build02/nixpkgs-update: use fetchTarball for updatescript fetcher

this is similar to how the github fetcher works

also update mkFetcher CacheDirectory

hosts/build02/nixpkgs-update.nix

@@ -126,7 +126,7 @@ let
       LogsDirectoryMode = "755";
       StateDirectory = "nixpkgs-update";
       StateDirectoryMode = "700";
-      CacheDirectory = "nixpkgs-update/worker";
+      CacheDirectory = "nixpkgs-update/fetcher";
       CacheDirectoryMode = "700";
     };
 

hosts/build02/packages-with-update-script.nix

@@ -1,5 +1,5 @@
 let
-  pkgs = import /var/cache/nixpkgs-update/worker/nixpkgs { };
+  pkgs = import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/master.tar.gz") { };
 in
 # code in the following let block was copied from nixos/nixpkgs under
   # the MIT License