slab: configure greetd for auto-login

this fucks with specialisations; Plasma enables greetd and tries to
evaluate the launch command for autologin when it is only set by
hyprland.nix and the Plasma specialisation doesn't define
nixfiles.programs.greetd.settings.command which makes it evaluate as
null

hosts/nullbox/configuration.nix

@@ -52,7 +52,6 @@
       profile.pc.enable = true;
       programs.adb.enable = true;
       programs.greetd = {
-        enable = true;
         preset = "tuigreet";
       };
       programs.unbound.enable = true;

hosts/slab/configuration.nix

@@ -60,11 +60,14 @@
       adb.enable = true;
       unbound.enable = true;
       greetd = {
-        enable = true;
         preset = "tuigreet";
       };
     };
   };
+  services.greetd.settings.initial_session = {
+    command = config.nixfiles.programs.greetd.settings.command;
+    user = "nullbite";
+  };
 
 
   networking.hostName = "slab";