try again with github workflow...

.github/workflows/matrix-commit-message.yml

@@ -6,7 +6,7 @@ jobs:
     name: Send message via Matrix
     steps:
       - run: |
-          commits=${{ join(github.event.commits.\*.id, ' ') }}
+          commits=${{ join(github.event.commits.*.id, ' ') }}
           echo '::set-output name=COMMIT_MESSAGE::$commits'
         id: commit-message
       - uses: narodnik/matrix-action@main