Upgrade to Rust 1.15.1 for build-geckolib
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

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