Add more permissions for pushing in prebuilt workflow (#30948)

Continuing on from https://github.com/commaai/openpilot/pull/30947
old-commit-hash: 221f81bbd3

.github/workflows/prebuilt.yaml

@@ -17,6 +17,8 @@ jobs:
       PUSH_IMAGE: true
     permissions:
       checks: read
+      contents: read
+      packages: write
     steps:
     - name: Wait for green check mark
       if: ${{ github.event_name != 'workflow_dispatch' }}