Enable GitHub actions cancel-in-progress for PRs

Pushing many commits to a pull request in a short amount of time can stall the
merge builds and also wastes energy unnecessarily. Enable concurrency to cancel
workflows of old commits in pull requests. Generate a common group name for pull
requests using github.event.pull_request.url with github.run_id as a fallback
for branches, which is unique and always available.

Closes GH-10799

.github/workflows/push.yml

@@ -17,6 +17,9 @@ on:
   pull_request:
     branches:
       - '**'
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.url || github.run_id }}
+  cancel-in-progress: true
 env:
   CC: ccache gcc
   CXX: ccache g++