Only install github extras when in a directory tracked by git

[oscarlevin]

As we move to supporting small documents, not all folders that we run pretext in will be tracked by git, so all the .devcontainer and workflow files are not needed.

[StevenClontz]

Seems reasonable - I want those in every project up on GitHub to facilitate collaboraiton and replicability, but obviously nothing is on GitHub unless tracked by git.