Old message:
read-only token in GH_TOKEN cannot be modified
This message was vague and some users did not understand that this
refers to the value that is read from environment variables.
New message:
$ GH_TOKEN=123 ghd auth login -h github.com
The value of the GH_TOKEN environment variable is being used for authentication.
To have GitHub CLI store credentials instead, first clear the value from the environment.
Adding GITHUB_TOKEN & GITHUB_ENTERPRISE_TOKEN support orthogonal to
Config was getting out of hand, especially in `auth` commands that
adjust their messaging and error status based on the presence of these
environment variables.
The new approach builds in support for tokens from environment straight
into Config object by composition. Thus, commands need not ever be
concerned with any specific environment variables.
