How do I specify in my `home-manager` configuration which channel to use? Currently `sudo nix-channel --list` have -
```
head.tar.gz https://github.com/NixOS/nixpkgs/archive/pull/100169/head.tar.gz           
home-manager https://github.com/rycee/home-manager/archive/master.tar.gz             
nixos https://nixos.org/channels/nixos-unstable
```