chore: Fix CI issue

Author: lqiu96

.github/workflows/downstream_protobuf_source_compatibility_check_nightly.yaml

@@ -1,5 +1,7 @@
-name: Downstream Source Compatibility Nightly
 on:
+  push:
+    branches:
+      - main
   workflow_dispatch:
     inputs:
       protobuf_versions:
@@ -9,9 +11,8 @@ on:
   pull_request:
   schedule:
     - cron: '0 1 * * *' # Nightly at 1am
-defaults:
-  run:
-    shell: bash
+
+name: Downstream Source Compatibility Nightly
 jobs:
   downstream-protobuf-source-test:
     runs-on: ubuntu-22.04