Trying to get the unstable
neovim, I found this
let
unstable = import (fetchTarball https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz) { };
in
{ nixpkgs ? import <nixpkgs> {} }:
with nixpkgs; mkShell {
buildInputs = [ hello unstable.neovim nixfmt ];
}