book: Install helix declaratively from nixpkgs on NixOS

This is really the most simple way, and helix is well maintained in
nixpkgs.

The flake solution is overly complicated for newcomers to NixOS, as I
just witnessed on our support room. Copying/adapting/understanding that
expression is far easier.

book/src/install.md

@@ -82,7 +82,34 @@ in the AUR, which builds the master branch.
 
 ### NixOS
 
-Helix is available as a [flake](https://nixos.wiki/wiki/Flakes) in the project
+Helix is packaged into nixpkgs and can be installed declaratively through your system configuration.
+
+```nix
+environment.systemPackages = with pkgs; [
+  helix
+];
+```
+
+If the version on the NixOS stable release is too old, an overlay can be used to update to the package
+version from `nixos-unstable`.
+
+```sh
+nix-channel --add https://channels.nixos.org/nixos-unstable nixos-unstable
+nix-channel --update
+```
+
+```nix
+let
+  unstable = import <nixos-unstable> { };
+in
+  nixpkgs.overlays = [
+    (prev: final: {
+      inherit (unstable) helix;
+    })
+  ];
+```
+
+Helix is also available as a [flake](https://nixos.wiki/wiki/Flakes) in the project
 root. Use `nix develop` to spin up a reproducible development shell. Outputs are
 cached for each push to master using [Cachix](https://www.cachix.org/). The
 flake is configured to automatically make use of this cache assuming the user