ci: Skip the Unity tests if the PR is an external contribution

We don't want to leak the license information.
Alternatively, we could just make the check non-required.

Author: kim

.github/workflows/csharp-test.yml

@@ -140,3 +140,6 @@ jobs:
           UNITY_EMAIL: ${{ secrets.UNITY_EMAIL }}
           UNITY_PASSWORD: ${{ secrets.UNITY_PASSWORD }}
           UNITY_SERIAL: ${{ secrets.UNITY_SERIAL }}
+        # Skip if this is an external contribution.
+        # The license secrets will be empty, so the step would fail anyway.
+        if: !github.event.pull_request.head.repo.fork