ci: See if this only runs one time in PRs

If we keep this, credit goes to @DanTup

See also:
- https://github.community/t/duplicate-checks-on-push-and-pull-request-simultaneous-event/18012/6
- Dart-Code/Dart-Code@612732d

Author: tony

.github/workflows/tests.yml

@@ -4,6 +4,8 @@ on: [push, pull_request]
 
 jobs:
   build:
+    # Don't run twice for internal PRs from our own repo
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
 
     runs-on: ubuntu-latest
     strategy: