Fix mutation test in CI by passing env vars correctly
Fix mutation test in CI by passing env vars correctly

Originally, buildbot would try to directly execute `RUSTFLAGS=` as a binary
since it was the first token in the command string, which won't work.

Fix this by using the `env` command to set the environment variables,
as there is currently no support for setting per-step environment variables
in the `buildbot_steps.yml` file.

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