chore: steal arg_v.value from copied arg in unpacking_collector (#4219)

include/pybind11/cast.h

@@ -1545,7 +1545,7 @@ private:
             throw cast_error_unable_to_convert_call_arg(a.name, a.type);
 #endif
         }
-        m_kwargs[a.name] = a.value;
+        m_kwargs[a.name] = std::move(a.value);
     }
 
     void process(list & /*args_list*/, detail::kwargs_proxy kp) {