gh-token: Support fetching token from GH_TOKEN and GITHUB_TOKEN envvars

[John T. Wodder II]

I would like to request that gh-token gain the ability (either by changing gh_token::get() or by adding a separate function) to check the GH_TOKEN and GITHUB_TOKEN envvars (in that order) for a GitHub token before falling back to reading from the gh config file, just like gh does. This would allow gh-token-enabled programs to run in environments like GitHub Actions where the GitHub token is typically passed as an environment variable.