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

[Nikolas Klauser via llvm-commits]

philnik777 wrote:

> > Can you limit the total number of jobs to 40? We need to have some spare capacity for running some of the other jobs, like the PR labeler, code_formatter, etc. We may be able to bump it higher in the future, but I think 40 is a good start.
> 
> I thought this was possible, but I haven't found a way to actually limit the number of jobs. I f you know how to implement this then that is great, but otherwise do you have an estimate of the number of concurrent machines you will need to handle the peak load?

Hm, yeah. I thought it could be done with `max-parallel:`, but that only limits the parallelism per PR. I'd expect a maximum of ~10 runs/hour for libc++, which would be 80 runners. That would only be the case once in a blue moon though. A lot of the work on libc++ is done off-peak and we generally don't throw out a bunch of PRs at once. If this actually becomes a problem we could also consider moving only some of the jobs to GitHub or limiting the number of parallel jobs to 4/PR.

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