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

default.nix

@@ -22,9 +22,11 @@ in
   config = {
     _module.args.owoLib = pkgs.callPackage ./lib { };
 
-    nixowos = lib.mkIf cfg.enable {
-      cli.enable = true;
-      system.enable = true;
-    };
+    nixowos = lib.mkIf cfg.enable (
+      lib.mkDefault {
+        cli.enable = true;
+        system.enable = true;
+      }
+    );
   };
 }