ci: Allow the nightly source compatibility check to be run manually

Author: lqiu96

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -14,8 +14,11 @@ on:
 name: Downstream Protobuf Compatibility Check Nightly
 jobs:
   downstream-protobuf-test:
-    # Checks if PR comes from Release-Please branch or if invoked from nightly job
-    if: github.head_ref == 'release-please--branches--main' || github.event_name == 'schedule'
+    # This job runs if any of the three conditions match:
+    # 1. PR is raised from Release-Please (PR comes from branch: release-please--branches-{branch_name})
+    # 2. Job is invoked by the nightly job (scheduled event)
+    # 3. Job is manually invoked via Github UI (workflow_dispatch event)
+    if: github.head_ref == 'release-please--branches--main' || github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: false