Don't update to the latest ref on non-master branches

.github/workflows/update_protobufs.yml

@@ -12,6 +12,7 @@ jobs:
           submodules: true
 
       - name: Update submodule
+        if: ${{ github.ref == 'refs/heads/master' }}
         run: |
           git submodule update --remote protobufs