ci: sign commits

Author: henryiii

.github/workflows/bump.yml

@@ -33,3 +33,4 @@ jobs:
             PR generated by "Update dependencies" [workflow](https://github.com/${{github.repository}}/actions/runs/${{github.run_id}}).
           token: ${{ steps.generate-token.outputs.token }}
           delete-branch: true
+          sign-commits: true