Trigger builds on main branch push

This is tricky. I wish we could trigger on all branches push, but it seems like internal PRs are run twice when using this:

```yaml

on:
  pull_request: {}
  push: {}
```

So specifying a branch is a bit better somehow. Related discussions:
* https://github.community/t/how-to-run-a-workflow-only-once-even-when-triggered-mutliple-times/16436
* https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: Unity Actions
 
 on:
   pull_request: {}
-  push: {}
+  push: { branches: [main] }
 
 env:
   UNITY_LICENSE: ${{ secrets.UNITY_LICENSE }}