.github/workflows/merge-staging.yml: change condition to `repository_owner`

This is consistent with the other actions.

.github/workflows/merge-staging.yml

···
 
 jobs:
   sync-branch:
-    if: github.repository == 'NixOS/nixpkgs'
+    if: github.repository_owner == 'NixOS'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2