Auto merge of #15526 - servo:1.15, r=Manishearth
Auto merge of #15526 - servo:1.15, r=Manishearth

Upgrade to Rust 1.15.1 for build-geckolib

Firefox CI already does:

https://bugzilla.mozilla.org/show_bug.cgi?id=1338311
https://bugzilla.mozilla.org/show_bug.cgi?id=1337153

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upsuper/servo,larsbergstrom/servo,upsuper/servo,szeged/servo,KiChjang/servo,mbrubeck/servo,rnestler/servo,mbrubeck/servo,nnethercote/servo,avadacatavra/servo,dsandeephegde/servo,CJ8664/servo,thiagopnts/servo,jimberlage/servo,paulrouget/servo,KiChjang/servo,paulrouget/servo,thiagopnts/servo,peterjoel/servo,rnestler/servo,ConnorGBrewster/servo,upsuper/servo,emilio/servo,splav/servo,SimonSapin/servo,avadacatavra/servo,fiji-flo/servo,KiChjang/servo,ConnorGBrewster/servo,emilio/servo,cbrewster/servo,anthgur/servo,avadacatavra/servo,KiChjang/servo,DominoTree/servo,thiagopnts/servo,eddyb/servo,cbrewster/servo,dsandeephegde/servo,larsbergstrom/servo,pyfisch/servo,KiChjang/servo,pyfisch/servo,avadacatavra/servo,notriddle/servo,jimberlage/servo,CJ8664/servo,DominoTree/servo,paulrouget/servo,fiji-flo/servo,nnethercote/servo,larsbergstrom/servo,peterjoel/servo,eddyb/servo,jimberlage/servo,DominoTree/servo,notriddle/servo,KiChjang/servo,canaltinova/servo,rnestler/servo,saneyuki/servo,anthgur/servo,avadacatavra/servo,eddyb/servo,saneyuki/servo,thiagopnts/servo,fiji-flo/servo,anthgur/servo,upsuper/servo,jimberlage/servo,splav/servo,dati91/servo,emilio/servo,upsuper/servo,szeged/servo,ConnorGBrewster/servo,saneyuki/servo,jimberlage/servo,ConnorGBrewster/servo,anthgur/servo,dati91/servo,pyfisch/servo,fiji-flo/servo,dsandeephegde/servo,mattnenterprise/servo,dati91/servo,emilio/servo,KiChjang/servo,sadmansk/servo,sadmansk/servo,nnethercote/servo,paulrouget/servo,SimonSapin/servo,DominoTree/servo,notriddle/servo,mbrubeck/servo,anthgur/servo,mbrubeck/servo,dati91/servo,larsbergstrom/servo,CJ8664/servo,DominoTree/servo,eddyb/servo,emilio/servo,dsandeephegde/servo,peterjoel/servo,DominoTree/servo,mbrubeck/servo,fiji-flo/servo,pyfisch/servo,paulrouget/servo,cbrewster/servo,saneyuki/servo,peterjoel/servo,eddyb/servo,SimonSapin/servo,canaltinova/servo,nnethercote/servo,canaltinova/servo,sadmansk/servo,canaltinova/servo,mattnenterprise/servo,cbrewster/servo,emilio/servo,rnestler/servo,SimonSapin/servo,splav/servo,larsbergstrom/servo,SimonSapin/servo,splav/servo,sadmansk/servo,mattnenterprise/servo,thiagopnts/servo,mattnenterprise/servo,splav/servo,mattnenterprise/servo,thiagopnts/servo,pyfisch/servo,DominoTree/servo,splav/servo,szeged/servo,saneyuki/servo,CJ8664/servo,fiji-flo/servo,CJ8664/servo,peterjoel/servo,mbrubeck/servo,larsbergstrom/servo,mattnenterprise/servo,larsbergstrom/servo,canaltinova/servo,szeged/servo,saneyuki/servo,notriddle/servo,dsandeephegde/servo,splav/servo,pyfisch/servo,szeged/servo,paulrouget/servo,saneyuki/servo,notriddle/servo,emilio/servo,jimberlage/servo,saneyuki/servo,SimonSapin/servo,notriddle/servo,thiagopnts/servo,sadmansk/servo,thiagopnts/servo,jimberlage/servo,szeged/servo,notriddle/servo,dati91/servo,upsuper/servo,peterjoel/servo,upsuper/servo,splav/servo,saneyuki/servo,peterjoel/servo,peterjoel/servo,canaltinova/servo,cbrewster/servo,SimonSapin/servo,SimonSapin/servo,cbrewster/servo,rnestler/servo,rnestler/servo,peterjoel/servo,mbrubeck/servo,notriddle/servo,DominoTree/servo,mattnenterprise/servo,ConnorGBrewster/servo,sadmansk/servo,nnethercote/servo,dsandeephegde/servo,peterjoel/servo,paulrouget/servo,nnethercote/servo,notriddle/servo,DominoTree/servo,larsbergstrom/servo,emilio/servo,KiChjang/servo,notriddle/servo,canaltinova/servo,avadacatavra/servo,dsandeephegde/servo,dati91/servo,cbrewster/servo,canaltinova/servo,pyfisch/servo,sadmansk/servo,mattnenterprise/servo,paulrouget/servo,pyfisch/servo,CJ8664/servo,emilio/servo,emilio/servo,szeged/servo,rnestler/servo,dsandeephegde/servo,KiChjang/servo,eddyb/servo,avadacatavra/servo,ConnorGBrewster/servo,saneyuki/servo,avadacatavra/servo,paulrouget/servo,ConnorGBrewster/servo,anthgur/servo,eddyb/servo,nnethercote/servo,pyfisch/servo,dati91/servo,splav/servo,upsuper/servo,sadmansk/servo,CJ8664/servo,eddyb/servo,ConnorGBrewster/servo,anthgur/servo,nnethercote/servo,nnethercote/servo,larsbergstrom/servo,mbrubeck/servo,szeged/servo,KiChjang/servo,jimberlage/servo,CJ8664/servo,jimberlage/servo,szeged/servo,rnestler/servo,fiji-flo/servo,pyfisch/servo,DominoTree/servo,larsbergstrom/servo,fiji-flo/servo,paulrouget/servo,szeged/servo,nnethercote/servo,anthgur/servo,splav/servo,dati91/servo,cbrewster/servo