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"
         ];
       };