Move debugging variable into useEffect (#50210)

## What?

Ensures the debugging variables are only assigned when the values
change.

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packages/next/src/client/components/app-router.tsx

@@ -331,17 +331,24 @@ function Router({
     return routerInstance
   }, [dispatch, navigate])
 
-  // Add `window.nd` for debugging purposes.
-  // This is not meant for use in applications as concurrent rendering will affect the cache/tree/router.
-  if (typeof window !== 'undefined') {
+  useEffect(() => {
+    // Exists for debugging purposes. Don't use in application code.
+    if (window.next) {
+      window.next.router = appRouter
+    }
+  }, [appRouter])
+
+  useEffect(() => {
+    // Add `window.nd` for debugging purposes.
+    // This is not meant for use in applications as concurrent rendering will affect the cache/tree/router.
     // @ts-ignore this is for debugging
     window.nd = {
       router: appRouter,
       cache,
       prefetchCache,
       tree,
     }
-  }
+  }, [appRouter, cache, prefetchCache, tree])
 
   // When mpaNavigation flag is set do a hard navigation to the new url.
   // Infinitely suspend because we don't actually want to rerender any child