[libcxx] [llvm] [libc++] Use GitHub-provided runners for the windows CI (PR #79326)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo Could you take a look here? I've tried a bunch of stuff, but couldn't really get things to run properly.

Sure, I can try to set this up. I have tested running libcxx tests on Windows GitHub Actions runners before, and got it working before (for a subset of configs, not using the libcxx `run-buildbot` script) - I'll see what I can do.

https://github.com/llvm/llvm-project/pull/79326