fix zaatar build: move printer groups to configs/default.nix

printing.nix can't conditionally set users.users.me.extraGroups —
any mention of users.users.me in the module system creates the user
entry and triggers the isNormalUser/isSystemUser assertion on machines
without a 'me' user (like zaatar).

Move lp/scanner groups to configs/default.nix where users.users.me is
defined. On zaatar, root gets lp/scanner groups directly.

configs/default.nix

@@ -66,6 +66,8 @@ in
         extraGroups = [
           "pipewire"
           "audio"
+          "lp"
+          "scanner"
         ];
       };
 

configs/printing.nix

@@ -26,7 +26,7 @@ in
     openFirewall = true;
   };
 
-  users.users.me.extraGroups = [ "lp" "scanner" ];
+  # users.users.me.extraGroups is set in configs/default.nix which defines the "me" user
 
   hardware.printers.ensurePrinters = [
     {