Hotfix for caffe2 windows build (#20417)
Summary:
We don't need to overlay vc env when not using ninja. CMake will deal with it automatically. Overlaying is a no-op when the env is the same with the generator specified but will generate the error "Cannot find CMAKE_CXX_COMPILER" when they are different.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20417
Differential Revision: D15317081
Pulled By: ezyang
fbshipit-source-id: 5d9100321ecd593e810c31158f22c67d3e34973b
