ci: rework GitHub caching strategy

[davidhewitt]

I realised today that we are facing many cache misses in CI because we skip running workflows on main, and GitHub Actions caches only populate from the default branch or the current branch.

So the vast majority of jobs will be cache misses until their branch is populated, because the default branch (main) runs nothing.

This PR therefore reworks things in an attempt to put some oomph back into our build times:

• I've set jobs to run on main just as they would in the merge queue
• I've removed the hardcoded keys to Swatinem/rust-cache, which shouldn't be necessary based on what it uses by default (job ID + OS + rust version)
• I've added two things to reduce cache thrashing:
  • A new CI Cache Cleanup workflow which is a direct copy of https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#force-deleting-cache-entries, to eagerly purge branch caches when a PR gets merged or closed.
  • save-if setting to Swatinem/rust-cache to avoid uploading caches from the merge queue, as that will produce a bulk of largely useless cache.

If this looks like it builds, I'll probably merge it right away without review and hope it speeds up CI times. If it creates problems we can easily revert.