Fix new PR cancel previous PR's workflow (#918)

see
https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/control-the-concurrency-of-workflows-and-jobs#example-only-cancel-in-progress-jobs-on-specific-branches

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ on:
 
 # One active job per PR, cancel older ones on push
 concurrency:
-  group: ${{ github.head_ref || github.run_id }}
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 # NOTE: because matrix cannot easily be shared across jobs, it should be copied to each job