Integrate nix-index

This is a try, not sure if I need it, it probably will increase consumed
time by update procedure.

home/programs.nix

@@ -4,6 +4,13 @@
   # Let Home Manager install and manage itself.
   home-manager.enable = true;
 
+  # Add hook to zsh and bash.
+  # Does not work as there is no sqlite file.
+  # command-not-found.enable = true;
+  # Use nix-index as replacement.
+  # This needs to build index, which I do in custom-update-system
+  nix-index.enable = true;
+
   neovim = import ./programs/neovim.nix {
     inherit pkgs;
   };

overlays/custom-update-system/default.nix

@@ -9,6 +9,7 @@ self: super:
       home-manager switch
       home-manager expire-generations '-30 days'
       nix-env --delete-generations +5
+      nix-index
       nix store gc
       nix store optimise
     '';