Without the need to use additional infrastructure, what is the simplest and most maintainable method...

GitHub Actions supports the use of service containers, which allows you to spin up a PostgreSQL database (or any other service) in a Docker container during your workflow. You can pull a PostgreSQL image from Docker Hub, and the container will automatically be available to your workflow job. This method requires no additional infrastructure and is easy to configure and maintain, as you simply define the container in the workflow file.