Auto-cancel older PR nox runs. (#1268)

.github/workflows/nox.yml

@@ -15,6 +15,11 @@ name: nox
     - cron: '0 9 * * *'
   workflow_dispatch:
 
+concurrency:
+  # Make sure there is at most one active run per PR, but do not cancel any non-PR runs
+  group: ${{ github.workflow }}-${{ (github.head_ref && github.event.number) || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   nox:
     uses: ansible-community/antsibull-nox/.github/workflows/reusable-nox-run.yml@main