CI: Skip expensive tests suites on push in forks and mirrors (#110074)

Skip expensive tests suites on push in forks and mirrors

.github/workflows/pr-e2e-tests.yml

@@ -20,6 +20,8 @@ env:
 
 jobs:
   detect-changes:
+    # Run on `grafana/grafana` main branch, or on pull requests to prevent double-running on mirrors
+    if: (github.event_name == 'pull_request' || (github.event_name == 'push' && github.repository == 'grafana/grafana'))
     name: Detect whether code changed
     runs-on: ubuntu-latest
     permissions: