Update RTW to use the `git` scheme

The `git` scheme was added to [Rose Documentation: File Creation Mode](https://metomi.github.io/rose/doc/html/api/configuration/file-creation.html) in [Rose v2.3.0](https://github.com/metomi/rose/releases/tag/2.3.0). Update the RTW to use File Creation Mode with the `git` scheme rather than directly cloning ESMValCore and ESMValTool.