Put _virtualenv in the build dir, not the objdir
auto merge of #2163 : Manishearth/servo/wptfix, r=Ms2ger

#2162 was a tad off, it moves the virtualenv to the object directory (x86_64-unknown-linux-gnu, etc) instead of the build directory (build/, or whatever you're using). This moves it back. (Otherwise the `clean-wpt` target won't work)