Fast shutdown for cancelled jobs

[haines]

We have our pull request builds set up with concurrency groups that cancel in-progress jobs to avoid wasting runner minutes on superseded commits.

When builds are cancelled, the cleanup step often takes several minutes to prune the BuildKit cache (which is weird because it never seems to take more than a few seconds on builds that run to completion). Sometimes the buildkitd graceful shutdown times out, as well.

I think it would be nice if there was a "fast shutdown" path for cancelled jobs which doesn't prune the cache, SIGKILLs buildkitd, and skips committing the sticky disk.