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

[Tom Stellard via llvm-commits]

tstellar 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?  

> We're also running the CI on PRs as if they're merged into `main`, so I think a cache from a recent `main` would have a much higher hit rate than a PR-specific cache from even a day ago.

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.

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