[support-infra] Fix duplicate GitHub Action running on PR comments

Cherry-pick mui/mui-public#1230

Author: oliviertassinari

.github/workflows/mark-duplicate.yml

@@ -9,6 +9,7 @@ permissions: {}
 jobs:
   mark-duplicate:
     runs-on: ubuntu-latest
+    if: ${{ !github.event.issue.pull_request }}
     permissions:
       contents: read
       issues: write