Mark Windows CPU jobs as unstable (#100581)
Caused by https://github.com/pytorch/pytorch/pull/100377, something removes VS2019 installation on the non-ephemeral runner. I think moving this to unstable is nicer to gather signals in trunk without completely disable the job or revert https://github.com/pytorch/pytorch/pull/100377 (for the Nth times)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100581
Approved by: https://github.com/clee2000, https://github.com/malfet
