[llvm] workflows/premerge: Generate a ccache artifact for each pull request (PR #124311)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Do we need separate actions for this? I thought it was possible to implement this by attaching the PR (if we're running against a pr and the number is available) to the cache key, and then setting the ccache action to also look for cache keys without a PR (like the most recent run against `main`)?

https://github.com/llvm/llvm-project/pull/124311