Replace zet with the z monolith

Signed-off-by: Oliver Davies <oliver@oliverdavies.uk>

modules/scripts/start-tmux.nix

@@ -1,4 +1,4 @@
-{ withSystem, ... }:
+{ inputs, withSystem, ... }:
 
 let
   name = "start-tmux";
@@ -10,9 +10,9 @@ in
       packages.${name} = pkgs.writeShellApplication {
         inherit name;
 
-        # TODO: Add `z` once its available as a package.
         runtimeInputs = with pkgs; [
           fzf
+          inputs.z.packages.${pkgs.system}.default
         ];
 
         text = ''