nix-shell -I nixpkgs=channel:nixos-unstable -p hugo works. This channel: syntax seems to only be documented well under docs for the new nix CLI.
Also, if you have flakes enabled, you can do e.g. nix shell github:NixOS/nixpkgs/nixos-unstable#hugo. You can also try nix shell nixpkgs#hugo, since the default for nixpkgs in the registry is nixpkgs-unstable:
$ nix registry list | grep flake:nixpkgs
global flake:nixpkgs github:NixOS/nixpkgs/nixpkgs-unstable
but if your system is using a flake, that might point to the nixpkgs for that flake instead.