Cancel redundant CI jobs per ref name

For non pull requests, we now cancel older CI jobs for the same name
instead of the same commit. This way pushing 4 commits two e.g. `main`
with a bit of time in between won't result in 4 CI pipelines, of which
the first 3 are redundant at that point.

.github/workflows/push.yml

@@ -9,7 +9,7 @@ on:
   pull_request:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.ref_name }}
   cancel-in-progress: true
 
 env: