If `HOMEBREW_GITHUB_API_TOKEN` is not set and `GITHUB_TOKEN` is: let's
use it. `GITHUB_TOKEN` is a somewhat standard env var to set for
GitHub authentication tokens (e.g. used by `hub`).
cli/parser but for environment variables. This makes it easier to set,
check, document and set defaults for environment variables in a single
location that can be used for query and documentation generation.
