So you want to commit changes generated by a GitHub Actions workflow back to your repo, and have that commit signed automatically?
Here's one way this is possible, using the REST API, the auto-generated GITHUB_TOKEN, and the GitHub CLI, gh, which is pre-installed on GitHub's hosted Actions runners.
You don't have to configure the git client, just add a step like the one below... Be sure to edit FILE_TO_COMMIT and DESTINATION_BRANCH to suit your needs.