Don't activate vc env again for cuda with ninja on Windows (#33700)
Summary:
Possibly get rid of https://github.com/pytorch/pytorch/issues/28271, https://github.com/pytorch/pytorch/issues/27463 and https://github.com/pytorch/pytorch/issues/25393.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33700
Differential Revision: D20089251
Pulled By: ezyang
fbshipit-source-id: 0cfe62b869fb874e25f06894aa76fadc44cf6817
