Remove python/_virtualenv/ dir in clean_build_artifacts.sh
Remove python/_virtualenv/ dir in clean_build_artifacts.sh

KiChjang/servo,saneyuki/servo,splav/servo,KiChjang/servo,saneyuki/servo,KiChjang/servo,saneyuki/servo,saneyuki/servo,splav/servo,saneyuki/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,saneyuki/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,DominoTree/servo,splav/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,DominoTree/servo,splav/servo,saneyuki/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,saneyuki/servo,DominoTree/servo,saneyuki/servo,splav/servo,saneyuki/servo,KiChjang/servo,DominoTree/servo,splav/servo