treewide: use lib.mkDefault for "collateral" config

this allows the user to override settings without lib.mkForce,
since that should only be needed for potentially unadvisable changes

graphical/default.nix

@@ -23,7 +23,7 @@ in
   };
 
   config = lib.mkIf cfg.enable {
-    nixowos.${name} = {
+    nixowos.${name} = lib.mkDefault {
       plasma.enable = config.services.desktopManager.plasma6.enable;
       mpv.enable = true;
     };