Don't limit CI concurrency on main (#6807)

The CI workflow used a blanket `cancel-in-progress: true`, which makes
sense for PR runs (collapse to the latest commit) but means pushes to
`main` cancel each other when several PRs merge in quick succession.
There is no shared state between CI runs that would justify either
cancelling or serializing them — each run is independent — and we'd
rather see every commit on `main` get a full check.

Make the concurrency group unique per `run_id` for non-PR events so
pushes to `main` neither cancel nor queue, while PRs keep the existing
cancel-on-new-push behavior through the shared per-ref group.

Author: agners

.github/workflows/ci.yaml

@@ -13,8 +13,8 @@ env:
   MYPY_CACHE_VERSION: 1
 
 concurrency:
-  group: "${{ github.workflow }}-${{ github.ref }}"
-  cancel-in-progress: true
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name == 'pull_request' && 'pr' || github.run_id }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
 
 jobs:
   # Separate job to pre-populate the base dependency cache