Upload generated project as an Artifact in GitHub Actions

Our repo contents are hard to test, because we're dynamically generating code. It would be great to store the resulting repo created with the template in our CI workflow as an artifact to cross-check the generated output.

This would likely use the following workflow: https://docs.github.com/en/actions/tutorials/store-and-share-data. Edits should be made to our top-level `.github/` folder.