CI: use github.head_ref to group PR action runs (#26505)

* CI: use github.head_ref to group PR action runs

for push triggers, github.ref is set to the "branch or tag ref that was pushed"
for pull_request triggers, it is set to the "pull request merge branch" (master?)

github.head_ref is only set when the trigger is pull_request

https://docs.github.com/en/actions/learn-github-actions/contexts#github-context

* only check github.ref for push event

.github/workflows/selfdrive_tests.yaml

@@ -7,7 +7,7 @@ on:
   pull_request:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref != 'refs/heads/master' && github.ref || github.run_id }}-${{ github.event_name }}
+  group: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' && github.run_id || github.head_ref || github.ref }}-${{ github.workflow }}-${{ github.event_name }}
   cancel-in-progress: true
 
 env:

.github/workflows/tools_tests.yaml

@@ -7,7 +7,7 @@ on:
   pull_request:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref != 'refs/heads/master' && github.ref || github.run_id }}-${{ github.event_name }}
+  group: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' && github.run_id || github.head_ref || github.ref }}-${{ github.workflow }}-${{ github.event_name }}
   cancel-in-progress: true
 
 env: