Change review apps PR workflow to leave only one comment

.github/workflows/review_apps_on_pr_change.yml

@@ -76,10 +76,12 @@ jobs:
 
       - name: Comment on PR
         env:
+          COMMENT_MARKER: <!-- review apps on pr change -->
           GH_TOKEN: ${{ github.token }}
         run: |
-
           cat <<EOF > "${{runner.temp}}/pr-comment.md"
+          $COMMENT_MARKER
+
           :tada: A review copy of this PR has been deployed! You can reach it at ${{steps.deploy.outputs.REVIEW_APP_URL}}.
 
           It may take 5 minutes or so for the application to be fully deployed and working. If it still isn't ready
@@ -89,4 +91,9 @@ jobs:
           For more details please see the [review app wiki page](https://github.com/alphagov/forms-team/wiki/Review-apps)
           EOF
 
+          old_comment_ids=$(gh api "repos/{owner}/{repo}/issues/${{github.event.pull_request.number}}/comments" --jq 'map(select((.user.login == "github-actions[bot]") and (.body | startswith($ENV.COMMENT_MARKER)))) | .[].id')
+          for comment_id in $old_comment_ids; do
+            gh api -X DELETE "repos/{owner}/{repo}/issues/comments/${comment_id}"
+          done
+
           gh pr comment "${{github.event.pull_request.html_url}}" --body-file "${{runner.temp}}/pr-comment.md"