Before:
$ GH_ENTERPRISE_TOKEN="..." gh pr create
could not find hosts config: not found
Now:
$ GH_ENTERPRISE_TOKEN="..." gh pr create
set the GH_HOST environment variable to specify which GitHub host to use
Also amends `gh help environment` documentation to suggest the use of
GH_HOST when scripting operations with GitHub Enterprise repositories.
This ensures that while having git remotes to point to either
`github.com` or authenticated GHE instances, adding another git remote
pointing to an unrelated host won't change the remote resolution in any
way, even if the unrelated remote is called `upstream` or `github` (and
thus normally took precedence).
