fix(export): make static export work with worker_threads (#47784)

Fixes #46993.

The issue is introduced in #46705 by @ijjk 

Next.js uses `jest-worker` for multiprocessing. `jest-worker` supports both `child_process` and `worker_threads` modes. Next.js use `child_process` mode by default, but can also switch to using `worker_threads` mode with an experimental flag `config.experimental.workerThreads`.

In #46705, @ijjk applies a fix that works for the `child_process` mode, which breaks the `worker_threads` mode (`jest-worker`'s `NodeThreadWorker` interface doesn't have the private property `_child`), causing static HTML export to fail with the following error (#46993):

```
> Build error occurred
TypeError: Cannot read properties of undefined (reading 'on')
    at createWorker (/Users/[redacted]/node_modules/next/dist/lib/worker.js:32:31)
    at new Worker (/Users/[redacted]/node_modules/next/dist/lib/worker.js:42:9)
    at /Users/[redacted]/node_modules/next/dist/build/index.js:629:35
    at async Span.traceAsyncFn (/Users/[redacted]/node_modules/next/dist/trace/trace.js:79:20)
    at async Object.build [as default] (/Users/[redacted]/node_modules/next/dist/build/index.js:74:29)
```

The PR fixes that.

packages/next/src/lib/worker.ts

@@ -14,6 +14,7 @@ export class Worker {
       timeout?: number
       onRestart?: (method: string, args: any[], attempts: number) => void
       exposedMethods: ReadonlyArray<string>
+      enableWorkerThreads?: boolean
     }
   ) {
     let { timeout, onRestart, ...farmOptions } = options
@@ -44,18 +45,29 @@ export class Worker {
         (resolve) => (resolveRestartPromise = resolve)
       )
 
-      for (const worker of ((this._worker as any)._workerPool?._workers ||
-        []) as {
-        _child: ChildProcess
-      }[]) {
-        worker._child.on('exit', (code, signal) => {
-          // log unexpected exit if .end() wasn't called
-          if ((code || signal) && this._worker) {
-            console.error(
-              `Static worker unexpectedly exited with code: ${code} and signal: ${signal}`
-            )
-          }
-        })
+      /**
+       * Jest Worker has two worker types, ChildProcessWorker (uses child_process) and NodeThreadWorker (uses worker_threads)
+       * Next.js uses ChildProcessWorker by default, but it can be switched to NodeThreadWorker with an experimental flag
+       *
+       * We only want to handle ChildProcessWorker's orphan process issue, so we access the private property "_child":
+       * https://github.com/facebook/jest/blob/b38d7d345a81d97d1dc3b68b8458b1837fbf19be/packages/jest-worker/src/workers/ChildProcessWorker.ts
+       *
+       * But this property is not available in NodeThreadWorker, so we need to check if we are using ChildProcessWorker
+       */
+      if (!farmOptions.enableWorkerThreads) {
+        for (const worker of ((this._worker as any)._workerPool?._workers ||
+          []) as {
+          _child?: ChildProcess
+        }[]) {
+          worker._child?.on('exit', (code, signal) => {
+            // log unexpected exit if .end() wasn't called
+            if ((code || signal) && this._worker) {
+              console.error(
+                `Static worker unexpectedly exited with code: ${code} and signal: ${signal}`
+              )
+            }
+          })
+        }
       }
 
       this._worker.getStdout().pipe(process.stdout)