extension_rollout: Prevent dispatch from non-main branches (#49396)

Release Notes:

- N/A

.github/workflows/extension_workflow_rollout.yml

@@ -7,6 +7,7 @@ on:
   workflow_dispatch: {}
 jobs:
   fetch_extension_repos:
+    if: (github.repository_owner == 'zed-industries' || github.repository_owner == 'zed-extensions') && github.ref == 'refs/heads/main'
     runs-on: namespace-profile-2x4-ubuntu-2404
     steps:
     - id: list-repos