make MyArgs non-copyable

this seems to have weird effects

src/eval-args.hh

@@ -27,6 +27,7 @@ class MyArgs : virtual public nix::MixEvalArgs,
                                        .useRegistries = false,
                                        .allowUnlocked = false};
     MyArgs();
+    MyArgs(const MyArgs&) = delete;
 
     void parseArgs(char** argv, int argc);
 };