[CI] Single job on PRs only

[talregev]

Single job on PRs only.

From here:
https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-a-fallback-value

If you build the group name with a property that is only defined for specific events, you can use a fallback value. For example, github.head_ref is only defined on pull_request events. If your workflow responds to other events in addition to pull_request events, you will need to provide a fallback to avoid a syntax error. The following concurrency group cancels in-progress jobs or runs on pull_request events only; if github.head_ref is undefined, the concurrency group will fallback to the run ID, which is guaranteed to be both unique and defined for the run.

concurrency: 
  group: ${{ github.head_ref || github.run_id }}
  cancel-in-progress: true