Allow for private repos

[Tom-van-Woudenberg]

Now it fails as can be seen here: https://github.com/TeachBooks/mirror_teachbook_manual/actions/runs/9254768881

[severinbratus]

If TeachBooks has the GH Free plan, deploying privates repos to GH Pages is not accessible: https://docs.github.com/en/pages/getting-started-with-github-pages/about-github-pages

[Tom-van-Woudenberg]

Teachbooks has a GitHub Teams plan, so it should be possible :)

[Tom-van-Woudenberg]

Teachbooks has a GitHub Teams plan, so it should be possible :)

But the workflow does fail because it cannot seem to clone the repository