The benchmark-diff concurrency group cancels the smoke run when the label is added #1147

Where: .github/workflows/benchmark-diff.yml concurrency.

Current gap: Both the smoke job (opened/synchronize) and the full rerun (labeled) share one group with cancel-in-progress: true. On the throwaway PR #1142 the run started by opened was cancelled the moment the label was added, so a PR that changes the workflow and is labelled never completes its smoke run.

Repro:

gh run list --workflow benchmark-diff.yml   # opened run: cancelled; labeled run: in progress

Suggested approach: Put the event action (or the job name) in the group key so the two kinds of run cancel only their own kind.

Done when: a labelled PR shows a finished smoke run and a running full rerun.