Fix checkout to use push context instead of invalid pull_request context

The workflow triggers on push to main, but checkout was referencing
github.event.pull_request which doesn't exist on push events. Also
bumps actions/checkout from v3 to v4.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hua7450

.github/workflows/update-versioning.yml

@@ -25,10 +25,9 @@ jobs:
           app-id: ${{ secrets.APP_ID }}
           private-key: ${{ secrets.APP_PRIVATE_KEY }}
       - name: Checkout repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
-          repository: ${{ github.event.pull_request.head.repo.full_name }}
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: main
           token: ${{ steps.app-token.outputs.token }}
       - name: Setup Python
         uses: actions/setup-python@v2