Fix virtualenv builds on Windows (#29273)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/29058.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29273
Differential Revision: D18349822
Pulled By: ezyang
fbshipit-source-id: c4d76521cc0742d890f22f1d7f32dede5600b651
