ci: a newer push cancels the check still running for an older one

Two pushes a few minutes apart ran two checks; the older, already
superseded, still emailed its result. Now a newer push to the same branch
cancels the check still running for the older one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

.github/workflows/check.yml

@@ -4,6 +4,11 @@ name: check
 on:
   push:
   pull_request:
+# A newer push to the same branch makes an older check pointless: it is
+# cancelled, so only the newest reports.
+concurrency:
+  group: check-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   check:
     runs-on: ubuntu-24.04