Fix follow-up action when a PR is closed (#847)

### Summary

When a submission PR is closed, an action posts a comment with
information for new ecosystem members. For example,
#823 (comment).

The problem is that this comment is posted even when the PR is closed
but wasn't merged. This PR fixes the workflow to only trigger when the
PR is merged.

.github/workflows/ecosystem-submission.yml

@@ -61,6 +61,7 @@ jobs:
     if: >
       startsWith(github.event.issue.title, '[Submission]:')
       && github.event.action == 'closed'
+      && github.event.pull_request.merged == true
       && contains(github.event.issue.labels.*.name, 'submission')
     runs-on: ubuntu-latest
     steps: