Support for only a single team drive

[Makeshift]

Because of #4, we dynamically create a union mount to avoid duplication, however the union mount requires >1 remote. If a user has a single team drive then creating the remote will fail, so we need to pick the destination the old way and avoid setting the env var.

[Makeshift]

This would be a problem if people are only capable of having a single team drive, eg. if they bought one off ebay or have been given access to one by a friend. The current 'getting started' assumes you're a gsuite admin, but there's no reason marauder wouldn't work without admin and on only a single team drive if configured slightly differently.