Use type rather than field presence

[pinch] / channels

[nixos]
channel_url = https://channels.nixos.org/nixos-20.03
git_repo = https://github.com/NixOS/nixpkgs.git
git_ref = nixos-20.03

[nixos-hardware]
git_repo = https://github.com/NixOS/nixos-hardware.git
git_ref = master