Use concurrency for GitHub Actions workflow (#2652)

This PR ensures that only the last execution per branch always works,
even when pushed multiple times in a row, for example. The older one is
canceled.

Refs: https://docs.github.com/en/actions/using-jobs/using-concurrency
https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/

Author: pirj

.github/workflows/ci.yml

@@ -10,6 +10,9 @@ on:
       - '*'
 permissions:
   contents: read
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   rubocop: