chore: only run preview on pull request

Author: noahnu

.github/workflows/ci.yaml

@@ -54,7 +54,7 @@ jobs:
     name: Preview
     runs-on: ubuntu-latest
     needs: [prepare]
-    if: "!contains(github.event.head_commit.message, '[skip ci]')"
+    if: "!contains(github.event.head_commit.message, '[skip ci]') && github.event_name == 'pull_request'"
     permissions:
       contents: read
       pull-requests: write