Don't assume the user's environment in mach_bootstrap.
Don't assume the user's environment in mach_bootstrap.

On Windows with multiple Pythons installed, this was causing python2.7
to bootstrap a 3.7 virtualenv that it couldn't make use of.

PIP_NAMES wasn't used at all, and VIRTUALENV_NAMES ends up being unused
now.

saneyuki/servo,emilio/servo,paulrouget/servo,larsbergstrom/servo,notriddle/servo,paulrouget/servo,pyfisch/servo,notriddle/servo,larsbergstrom/servo,DominoTree/servo,KiChjang/servo,splav/servo,nnethercote/servo,pyfisch/servo,pyfisch/servo,nnethercote/servo,splav/servo,splav/servo,notriddle/servo,nnethercote/servo,splav/servo,notriddle/servo,splav/servo,larsbergstrom/servo,paulrouget/servo,larsbergstrom/servo,paulrouget/servo,DominoTree/servo,larsbergstrom/servo,splav/servo,nnethercote/servo,nnethercote/servo,nnethercote/servo,DominoTree/servo,notriddle/servo,KiChjang/servo,emilio/servo,saneyuki/servo,saneyuki/servo,DominoTree/servo,saneyuki/servo,pyfisch/servo,KiChjang/servo,paulrouget/servo,saneyuki/servo,saneyuki/servo,pyfisch/servo,DominoTree/servo,splav/servo,KiChjang/servo,KiChjang/servo,pyfisch/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,DominoTree/servo,emilio/servo,emilio/servo,paulrouget/servo,pyfisch/servo,saneyuki/servo,larsbergstrom/servo,larsbergstrom/servo,nnethercote/servo,notriddle/servo,splav/servo,larsbergstrom/servo,nnethercote/servo,emilio/servo,notriddle/servo,emilio/servo,DominoTree/servo,KiChjang/servo,nnethercote/servo,paulrouget/servo,nnethercote/servo,splav/servo,pyfisch/servo,splav/servo,pyfisch/servo,DominoTree/servo,emilio/servo,KiChjang/servo,pyfisch/servo,notriddle/servo,KiChjang/servo,paulrouget/servo,KiChjang/servo,emilio/servo,emilio/servo,notriddle/servo,saneyuki/servo,larsbergstrom/servo,larsbergstrom/servo,paulrouget/servo,emilio/servo,paulrouget/servo,notriddle/servo,saneyuki/servo,saneyuki/servo