How do I set the default channel in NixOS's `/etc/configuration.nix`?

There is a command to set it and rebuild with

```
sudo nix-channel --add https://nixos.org/channels/nixpkgs-unstable
sudo nixos-rebuild switch -I nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixpkgs

```

but I'd like to have it setup in configuration.nix so I don't have to remember how to do this everytime. Thanks!