Fix cl.exe detection in cpu/fused_kernel.cpp (#50085)
Summary:
The command used here is essentially `where cl.exe`. By using `system()` we will not be able to find cl.exe unless we are using VS Developer Prompt, which makes `activate()` meaningless. Change `system()` to `run()` fixes this.
Found during https://github.com/pytorch/pytorch/issues/49781.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50085
Reviewed By: smessmer
Differential Revision: D25782054
Pulled By: ezyang
fbshipit-source-id: e8e3cac903a73f3bd78def667ebe0e93201814c8
