ci: cancel in-flight runs when a new push lands on the same PR

Add a concurrency block keyed on workflow + ref so a fresh push to a pull request automatically cancels any still-running prior run for that ref. cancel-in-progress is gated on the pull_request event, so master / release / check_suite triggers are left alone (they should never cancel themselves).

Author: grandixximo

.github/workflows/ci.yml

@@ -13,6 +13,10 @@ on:
 permissions:
   contents: read #  to fetch code (actions/checkout)
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 env:
   DEBIAN_FRONTEND: noninteractive