Use the correct free function to free plugin data.

The array was created with `mju_malloc` (which does an aligned alloc) and released with `free` (which doesn't care about alignment), which led to errors on Windows.

PiperOrigin-RevId: 578186714
Change-Id: I193987bed061e945c49d7268a9a9af3f17af362a

src/engine/engine_io.c

@@ -1232,7 +1232,7 @@ mjData* mj_copyData(mjData* dest, const mjModel* m, const mjData* src) {
   // restore plugin_data
   if (plugin_data_size) {
     memcpy(dest->plugin_data, save_plugin_data, plugin_data_size);
-    free(save_plugin_data);
+    mju_free(save_plugin_data);
     save_plugin_data = NULL;
   }