When there was a previously valid token that was granted some scopes,
ensure all those scopes will be re-requested when doing the
authentication flow for the new token.
This is to avoid hitting the filesystem and resolving symlinks
unnecessarily. The value of executable is just used conditionally by a
handful of commands.
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.
