ci: use signed commits for update workflow pull requests (#544)

Author: mayeut

.github/workflows/update-dependencies.yml

@@ -54,7 +54,6 @@ jobs:
 
           PR generated by "Update ${{ matrix.dependency_nice }}" [workflow](https://github.com/${{github.repository}}/actions/runs/${{github.run_id}}).
         branch: update-${{ matrix.dependency }}-pr
-        committer: "scikit-build-app-bot[bot] <173546081+scikit-build-app-bot[bot]@users.noreply.github.com>"
-        author: "scikit-build-app-bot[bot] <173546081+scikit-build-app-bot[bot]@users.noreply.github.com>"
+        sign-commits: true
         token: ${{ steps.app-token.outputs.token }}
         delete-branch: true