[llvm] Propagate the Buildkite pre-merge setup to GitHub Action (PR #81077)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

I don't: I was planning to setup a GitHub Action for the window CI that would reuse the buildkite script (just invoke it should be doable basically), but I haven't got to it yet. Do you want to give it a try?

https://github.com/llvm/llvm-project/pull/81077