ci: fix conf tests group concurrency key

`run_test_cases.yaml` and `run_conf_tests.yaml` seem to be cancelling
each other.

.github/workflows/run_conf_tests.yaml

@@ -1,7 +1,7 @@
 name: Run Configuration tests
 
 concurrency:
-  group: test-${{ github.event_name }}-${{ github.ref }}
+  group: conftest-${{ github.event_name }}-${{ github.ref }}
   cancel-in-progress: true
 
 on: