Only enable worker threads if there is more than 1 worker

Multiple worker threads can cause problems when certain dependencies are
being used, see e.g. vercel#7894

This patch allows disabling of worker threads by setting
`config.experimental.cpus = 1`.

The benefit of spawning 1 worker thread, if there is any at all, should
very limited anyways, so the workload can just as well be processed in
the main thread.

packages/next/build/index.ts

@@ -337,7 +337,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
 
   const staticCheckWorkers = new Worker(staticCheckWorker, {
     numWorkers: config.experimental.cpus,
-    enableWorkerThreads: true,
+    enableWorkerThreads: config.experimental.cpus > 1,
   })
 
   const analysisBegin = process.hrtime()

packages/next/build/webpack/plugins/terser-webpack-plugin/src/TaskRunner.js

@@ -28,8 +28,8 @@ export default class TaskRunner {
 
     if (this.maxConcurrentWorkers > 1) {
       this.workers = new Worker(worker, {
-        enableWorkerThreads: true,
         numWorkers: this.maxConcurrentWorkers,
+        enableWorkerThreads: this.maxConcurrentWorkers > 1,
       })
       this.boundWorkers = options => this.workers.default(options)
     } else {

packages/next/export/index.ts

@@ -267,7 +267,7 @@ export default async function(
     {
       maxRetries: 0,
       numWorkers: threads,
-      enableWorkerThreads: true,
+      enableWorkerThreads: threads > 1,
       exposedMethods: ['default'],
     }
   ) as any