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

[Nikolas Klauser via llvm-commits]

philnik777 wrote:

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

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