I cheated and did a stateful thing (because I couldn't figure out flakes): sudo nix-channel --add https://nixos.org/channels/nixos-unstable nixos-unstable

and then I added this blurb to my Nix config:

nixpkgs.config = { allowUnfree = true; packageOverrides = pkgs: { unstable = import <nixos-unstable> { config = config.nixpkgs.config; }; }; };