ci: the nightly and Run workflow get their own concurrency group (#877)

Only one run per concurrency group can wait. On a busy main each push replaced the waiting run, so the nightly and Run workflow runs on main were cancelled before they started. The group now includes the event.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

.github/workflows/ci.yml

@@ -21,8 +21,9 @@ permissions:
   contents: read
 
 concurrency:
-  group: ci-${{ github.ref }}
-  # Every push to main gets its full run; a new PR push replaces the old run.
+  # A new PR push replaces the old run. Only one run per group may wait, so a busy main would
+  # cancel the waiting nightly (or a Run workflow) before it starts: they get groups of their own.
+  group: ci-${{ github.event_name }}-${{ github.ref }}
   cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 env: