Currently we let a user get through two prompts of gh auth login when GITHUB_TOKEN is set, then unceremoniously tell them that the token is read-only.

We should proactively notice that situation when gh auth login is run and give clear messaging about unsetting the token in order to use the command.

Initially we thought only power users would be using GITHUB_TOKEN as an opt-in choice, making this fine; but in Codespaces GITHUB_TOKEN is being set without users ever knowing about it.