Following the NixOS installation guide I ended up with a setup using a stable NixOS channel that just keeps giving.Sometimes, however, it can be necessary to include packages from an unstable channel.

This is the snippet that you can use in your nix configuration if you want to install unstable neovim:

{ config, pkgs, ... }:
let
  unstableTarball =
    fetchTarball
      https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz;
in
{
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];
  nixpkgs.config = {
    packageOverrides = pkgs: with pkgs; {
      unstable = import unstableTarball {
        config = config.nixpkgs.config;
      };
    };
  };
  environment.systemPackages = with pkgs; [
    unstable.neovim
    emacs
  ];
}