Fix memory watcher reboot (#54760)

I've introduced a bug with memory watching in development in this PR: #53523.

Specifically when calling `cleanup` the process would exit because the `exit` listener on the childprocess would be called which then calls `process.exit(0)`. This caused a condition where calling `worker.end()` would immediately quit the process.

This PR ensures the cleanup listeners for childprocess are disabled (`.off(`) so that when we call `worker.end()` the process is terminated without the main process being terminated.

packages/next/src/cli/next-dev.ts

@@ -193,6 +193,12 @@ async function createRouterWorker(fullConfig: NextConfigComplete): Promise<{
   return {
     worker,
     cleanup: async () => {
+      // Remove process listeners for childprocess too.
+      for (const curWorker of ((worker as any)._workerPool?._workers || []) as {
+        _child?: ChildProcess
+      }[]) {
+        curWorker._child?.off('exit', cleanup)
+      }
       process.off('exit', cleanup)
       process.off('SIGINT', cleanup)
       process.off('SIGTERM', cleanup)