auto merge of #5200 : mbrubeck/servo/cache-dir, r=larsbergstrom
auto merge of #5200 : mbrubeck/servo/cache-dir, r=larsbergstrom

This will be set in servo/saltfs to prevent our buildbot builders from re-downloading the build tools for every build.

r? @larsbergstrom or @metajack or @Manishearth

s142857/servo,rixrix/servo,vks/servo,KiChjang/servo,pgonda/servo,nick-thompson/servo,GreenRecycleBin/servo,akosel/servo,DominoTree/servo,brendandahl/servo,notriddle/servo,larsbergstrom/servo,paulrouget/servo,thiagopnts/servo,j3parker/servo,tafia/servo,jdramani/servo,akosel/servo,tempbottle/servo,rentongzhang/servo,echochamber/servo,s142857/servo,hyowon/servo,jlegendary/servo,ryancanhelpyou/servo,snf/servo,splav/servo,sadmansk/servo,dsandeephegde/servo,pyfisch/servo,GyrosOfWar/servo,srbhklkrn/SERVOENGINE,dati91/servo,dsandeephegde/servo,aweinstock314/servo,nnethercote/servo,nerith/servo,fiji-flo/servo,mattnenterprise/servo,upsuper/servo,GreenRecycleBin/servo,huonw/servo,paulrouget/servo,deokjinkim/servo,GyrosOfWar/servo,wpgallih/servo,GyrosOfWar/servo,runarberg/servo,fiji-flo/servo,runarberg/servo,zhangjunlei26/servo,deokjinkim/servo,pyecs/servo,jgraham/servo,DominoTree/servo,KiChjang/servo,eddyb/servo,rixrix/servo,srbhklkrn/SERVOENGINE,jdramani/servo,emilio/servo,bjwbell/servo,avadacatavra/servo,anthgur/servo,larsbergstrom/servo,mattnenterprise/servo,thiagopnts/servo,vks/servo,pyfisch/servo,runarberg/servo,nerith/servo,meh/servo,dagnir/servo,samfoo/servo,WriterOfAlicrow/servo,wpgallih/servo,eddyb/servo,ConnorGBrewster/servo,evilpie/servo,dagnir/servo,hyowon/servo,szeged/servo,mbrubeck/servo,s142857/servo,mattnenterprise/servo,dati91/servo,peterjoel/servo,anthgur/servo,cbrewster/servo,aidanhs/servo,AnthonyBroadCrawford/servo,aweinstock314/servo,caldwell/servo,CJ8664/servo,nrc/servo,zentner-kyle/servo,youprofit/servo,tempbottle/servo,steveklabnik/servo,jlegendary/servo,snf/servo,boghison/servo,pyfisch/servo,A-deLuna/servo,pgonda/servo,steveklabnik/servo,eddyb/servo,GreenRecycleBin/servo,SimonSapin/servo,nnethercote/servo,emilio/servo,dagnir/servo,wartman4404/servo,steveklabnik/servo,avadacatavra/servo,SimonSapin/servo,notriddle/servo,snf/servo,youprofit/servo,caldwell/servo,avadacatavra/servo,SimonSapin/servo,zhangjunlei26/servo,upsuper/servo,boghison/servo,aweinstock314/servo,g-k/servo,ConnorGBrewster/servo,walac/servo,DominoTree/servo,GyrosOfWar/servo,SimonSapin/servo,cbrewster/servo,brendandahl/servo,ryancanhelpyou/servo,mattnenterprise/servo,splav/servo,peterjoel/servo,WriterOfAlicrow/servo,rixrix/servo,rentongzhang/servo,splav/servo,nnethercote/servo,upsuper/servo,dvberkel/servo,dhananjay92/servo,mt2d2/servo,SimonSapin/servo,meh/servo,pgonda/servo,nrc/servo,youprofit/servo,youprofit/servo,jlegendary/servo,dati91/servo,saratang/servo,huonw/servo,canaltinova/servo,evilpie/servo,nick-thompson/servo,aidanhs/servo,RenaudParis/servo,GreenRecycleBin/servo,zentner-kyle/servo,jgraham/servo,tempbottle/servo,canaltinova/servo,codemac/servo,sadmansk/servo,g-k/servo,larsbergstrom/servo,wartman4404/servo,GyrosOfWar/servo,g-k/servo,mattnenterprise/servo,WriterOfAlicrow/servo,brendandahl/servo,GreenRecycleBin/servo,pyfisch/servo,wartman4404/servo,thiagopnts/servo,mdibaiee/servo,KiChjang/servo,fiji-flo/servo,GyrosOfWar/servo,youprofit/servo,notriddle/servo,tschneidereit/servo,aweinstock314/servo,tafia/servo,mbrubeck/servo,wartman4404/servo,boghison/servo,mbrubeck/servo,ryancanhelpyou/servo,emilio/servo,DominoTree/servo,RenaudParis/servo,hyowon/servo,wartman4404/servo,SimonSapin/servo,mattnenterprise/servo,A-deLuna/servo,runarberg/servo,zhangjunlei26/servo,SimonSapin/servo,indykish/servo,splav/servo,samfoo/servo,indykish/servo,sadmansk/servo,saratang/servo,fiji-flo/servo,walac/servo,dvberkel/servo,huonw/servo,vks/servo,saneyuki/servo,szeged/servo,mt2d2/servo,srbhklkrn/SERVOENGINE,splav/servo,nnethercote/servo,larsbergstrom/servo,paulrouget/servo,srbhklkrn/SERVOENGINE,splav/servo,KiChjang/servo,deokjinkim/servo,saneyuki/servo,anthgur/servo,zhangjunlei26/servo,splav/servo,echochamber/servo,karlito40/servo,saneyuki/servo,dagnir/servo,meh/servo,echochamber/servo,KiChjang/servo,szeged/servo,cbrewster/servo,steveklabnik/servo,nrc/servo,saratang/servo,ConnorGBrewster/servo,jdramani/servo,upsuper/servo,pgonda/servo,dsandeephegde/servo,tafia/servo,upsuper/servo,wpgallih/servo,notriddle/servo,pyfisch/servo,pyfisch/servo,rnestler/servo,GreenRecycleBin/servo,rnestler/servo,dsandeephegde/servo,KiChjang/servo,emilio/servo,zhangjunlei26/servo,srbhklkrn/SERVOENGINE,tschneidereit/servo,larsbergstrom/servo,DominoTree/servo,evilpie/servo,DominoTree/servo,jgraham/servo,saneyuki/servo,nick-thompson/servo,RenaudParis/servo,jimberlage/servo,sadmansk/servo,avadacatavra/servo,tafia/servo,tempbottle/servo,GyrosOfWar/servo,ConnorGBrewster/servo,kindersung/servo,wartman4404/servo,walac/servo,evilpie/servo,codemac/servo,kindersung/servo,huonw/servo,brendandahl/servo,upsuper/servo,rixrix/servo,mdibaiee/servo,anthgur/servo,nick-thompson/servo,szeged/servo,echochamber/servo,hyowon/servo,s142857/servo,szeged/servo,anthgur/servo,jdramani/servo,larsbergstrom/servo,notriddle/servo,g-k/servo,eddyb/servo,fiji-flo/servo,A-deLuna/servo,peterjoel/servo,jimberlage/servo,dati91/servo,anthgur/servo,g-k/servo,indykish/servo,aweinstock314/servo,tempbottle/servo,saratang/servo,hyowon/servo,j3parker/servo,rnestler/servo,steveklabnik/servo,indykish/servo,zentner-kyle/servo,thiagopnts/servo,AnthonyBroadCrawford/servo,boghison/servo,jdramani/servo,dvberkel/servo,snf/servo,tafia/servo,mbrubeck/servo,aidanhs/servo,tempbottle/servo,dagnir/servo,jimberlage/servo,pyecs/servo,walac/servo,dati91/servo,g-k/servo,larsbergstrom/servo,nick-thompson/servo,jimberlage/servo,boghison/servo,caldwell/servo,dagnir/servo,ConnorGBrewster/servo,j3parker/servo,paulrouget/servo,indykish/servo,tschneidereit/servo,A-deLuna/servo,canaltinova/servo,tschneidereit/servo,pgonda/servo,mt2d2/servo,notriddle/servo,wpgallih/servo,j3parker/servo,pyecs/servo,codemac/servo,zhangjunlei26/servo,rnestler/servo,nnethercote/servo,bjwbell/servo,anthgur/servo,dvberkel/servo,kindersung/servo,youprofit/servo,zentner-kyle/servo,saratang/servo,ryancanhelpyou/servo,runarberg/servo,huonw/servo,rixrix/servo,fiji-flo/servo,tschneidereit/servo,nerith/servo,evilpie/servo,jimberlage/servo,akosel/servo,pyecs/servo,dhananjay92/servo,fiji-flo/servo,nerith/servo,emilio/servo,cbrewster/servo,kindersung/servo,saneyuki/servo,rnestler/servo,mbrubeck/servo,zhangjunlei26/servo,evilpie/servo,canaltinova/servo,WriterOfAlicrow/servo,DominoTree/servo,vks/servo,pyecs/servo,zhangjunlei26/servo,boghison/servo,cbrewster/servo,sadmansk/servo,karlito40/servo,nnethercote/servo,walac/servo,huonw/servo,huonw/servo,paulrouget/servo,tafia/servo,emilio/servo,steveklabnik/servo,paulrouget/servo,larsbergstrom/servo,samfoo/servo,AnthonyBroadCrawford/servo,GreenRecycleBin/servo,RenaudParis/servo,codemac/servo,RenaudParis/servo,saneyuki/servo,A-deLuna/servo,cbrewster/servo,deokjinkim/servo,codemac/servo,saneyuki/servo,nerith/servo,CJ8664/servo,dati91/servo,meh/servo,thiagopnts/servo,AnthonyBroadCrawford/servo,paulrouget/servo,aidanhs/servo,wpgallih/servo,kindersung/servo,srbhklkrn/SERVOENGINE,peterjoel/servo,KiChjang/servo,akosel/servo,zhangjunlei26/servo,jdramani/servo,pyecs/servo,aidanhs/servo,evilpie/servo,A-deLuna/servo,mattnenterprise/servo,nrc/servo,avadacatavra/servo,meh/servo,mbrubeck/servo,emilio/servo,runarberg/servo,mattnenterprise/servo,notriddle/servo,karlito40/servo,eddyb/servo,zentner-kyle/servo,AnthonyBroadCrawford/servo,CJ8664/servo,zentner-kyle/servo,wpgallih/servo,szeged/servo,dsandeephegde/servo,rnestler/servo,caldwell/servo,saneyuki/servo,samfoo/servo,KiChjang/servo,thiagopnts/servo,evilpie/servo,rixrix/servo,tschneidereit/servo,szeged/servo,kindersung/servo,indykish/servo,jlegendary/servo,mt2d2/servo,dvberkel/servo,wpgallih/servo,caldwell/servo,mdibaiee/servo,codemac/servo,kindersung/servo,nerith/servo,jgraham/servo,ryancanhelpyou/servo,rentongzhang/servo,meh/servo,szeged/servo,karlito40/servo,upsuper/servo,GreenRecycleBin/servo,s142857/servo,WriterOfAlicrow/servo,dhananjay92/servo,nick-thompson/servo,A-deLuna/servo,canaltinova/servo,aidanhs/servo,dati91/servo,paulrouget/servo,ConnorGBrewster/servo,WriterOfAlicrow/servo,peterjoel/servo,nnethercote/servo,bjwbell/servo,pyfisch/servo,nrc/servo,caldwell/servo,saneyuki/servo,mdibaiee/servo,mdibaiee/servo,nnethercote/servo,KiChjang/servo,walac/servo,jgraham/servo,nnethercote/servo,eddyb/servo,jimberlage/servo,AnthonyBroadCrawford/servo,mdibaiee/servo,bjwbell/servo,youprofit/servo,upsuper/servo,mt2d2/servo,brendandahl/servo,ConnorGBrewster/servo,peterjoel/servo,jimberlage/servo,deokjinkim/servo,akosel/servo,dvberkel/servo,splav/servo,CJ8664/servo,snf/servo,boghison/servo,rentongzhang/servo,jdramani/servo,aidanhs/servo,j3parker/servo,tempbottle/servo,cbrewster/servo,vks/servo,sadmansk/servo,notriddle/servo,g-k/servo,samfoo/servo,dhananjay92/servo,rentongzhang/servo,deokjinkim/servo,thiagopnts/servo,bjwbell/servo,canaltinova/servo,pyecs/servo,dhananjay92/servo,vks/servo,karlito40/servo,DominoTree/servo,caldwell/servo,splav/servo,mbrubeck/servo,notriddle/servo,eddyb/servo,mbrubeck/servo,pyfisch/servo,anthgur/servo,jimberlage/servo,dsandeephegde/servo,akosel/servo,CJ8664/servo,pgonda/servo,dagnir/servo,peterjoel/servo,samfoo/servo,indykish/servo,jimberlage/servo,WriterOfAlicrow/servo,j3parker/servo,s142857/servo,bjwbell/servo,snf/servo,hyowon/servo,wpgallih/servo,nrc/servo,runarberg/servo,steveklabnik/servo,j3parker/servo,rentongzhang/servo,larsbergstrom/servo,dsandeephegde/servo,SimonSapin/servo,rixrix/servo,echochamber/servo,samfoo/servo,wartman4404/servo,larsbergstrom/servo,s142857/servo,ryancanhelpyou/servo,jgraham/servo,karlito40/servo,vks/servo,avadacatavra/servo,CJ8664/servo,dsandeephegde/servo,karlito40/servo,zentner-kyle/servo,avadacatavra/servo,peterjoel/servo,rnestler/servo,pyfisch/servo,pgonda/servo,RenaudParis/servo,dhananjay92/servo,thiagopnts/servo,DominoTree/servo,paulrouget/servo,saneyuki/servo,codemac/servo,rixrix/servo,jgraham/servo,akosel/servo,nrc/servo,RenaudParis/servo,fiji-flo/servo,aweinstock314/servo,splav/servo,nerith/servo,eddyb/servo,saratang/servo,bjwbell/servo,rentongzhang/servo,mt2d2/servo,snf/servo,emilio/servo,paulrouget/servo,jlegendary/servo,mdibaiee/servo,rixrix/servo,rnestler/servo,peterjoel/servo,meh/servo,tschneidereit/servo,ConnorGBrewster/servo,indykish/servo,echochamber/servo,evilpie/servo,saratang/servo,ryancanhelpyou/servo,AnthonyBroadCrawford/servo,emilio/servo,CJ8664/servo,tafia/servo,brendandahl/servo,DominoTree/servo,hyowon/servo,brendandahl/servo,emilio/servo,mt2d2/servo,GreenRecycleBin/servo,dati91/servo,deokjinkim/servo,walac/servo,dvberkel/servo,aweinstock314/servo,szeged/servo,nnethercote/servo,echochamber/servo,avadacatavra/servo,canaltinova/servo,indykish/servo,jlegendary/servo,nick-thompson/servo,peterjoel/servo,wpgallih/servo,CJ8664/servo,dhananjay92/servo,jlegendary/servo,notriddle/servo,srbhklkrn/SERVOENGINE,KiChjang/servo,szeged/servo,cbrewster/servo,sadmansk/servo,canaltinova/servo,pyfisch/servo,sadmansk/servo