Fix CLI tool installation in GitHub for Mac

Symlinks the 'github' CLI binary that enables opening GitHub for Mac
from the command line. As per the defaults for the application, the
internal `github_cli` binary is symlinked simply as `github`.

Note this intentionally does *not* also symlink the bundled versions.
of `git` itself, as anyone using brewcask is going to want to be able
to manage those normally via homebrew.`