tools: github actions: prevent multiple concurrent workflow runs on t…

…he same branch/ref

- cancel other pending/in-progress workflows if a new commit is pushed to the ref
- ref. https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency

.github/workflows/ci.yml

@@ -8,6 +8,10 @@ on:
 env:
   GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
 
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test_run:
     runs-on: ubuntu-latest