[llvm] [workflows] Port buildkite Windows config to GitHub actions (PR #82093)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> @AaronBallman If you want to use them as GitHub Action self-hosted runners, I can help set that up.

Thanks! FWIW, we're willing to use them in whatever way the community will get the most value from, so we can be somewhat flexible on how to move forward.

https://github.com/llvm/llvm-project/pull/82093