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. github.head_ref is only set
for pull requests. For the main branches we fall back to github.run_id which is
unique and always available.

Author: iluuu1994

.github/workflows/push.yml

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