TokenForHost returns string "oauth_token" as source when config is read from file

TokenForHost `source` return value should be a value like GH_TOKEN or GITHUB_TOKEN when the token comes from an environment variable, or a file path (e.g. `/path/to/hosts.yml`) when the value comes from a file. However, a static string `oauth_token` is returned instead of a file name: https://github.com/cli/go-gh/blob/da64a13f361b2a1b9a1eb5358b7d7eee7506f6a2/pkg/auth/auth.go#L64

Since `oauth_token` source is not useful to find out where a value comes from, printing the file name would be better.

Ref. https://github.com/cli/go-gh/pull/44