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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> It looks like caches from PRs stored in the target repo cache: https://github.com/llvm/llvm-project/actions/caches, which now that I think about it, seems like a security risk to me. It also says we are using ~150GB/10GB of the cache, so I guess it's a soft limit?

Yeah, it's definitely a soft limit. Not sure what the actual logic is, but full eviction doesn't happen for some time.

> Yes, that's a good point. I wonder if we should limit the PRs to read-only access to the cache, that way it won't be polluted with lots of different PR builds.

This seems like the best course of action to me. The only other thing we would need to figure out is special casing the release branch, because main and the release branch will diverge quickly. Just adding a key to the cache on the target branch wouldn't be a bad first step, but I'm guessing those cache keys will get evicted pretty quickly.

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