Correct build workers perf issue (#51271)

This continues off of #49937 and
#50194 ensuring we don't regress
on build perf due to reducing the amount of workers being used in
parallel.

Fixes: #51201
Fixes: #50555

packages/next/src/build/index.ts

@@ -1141,10 +1141,6 @@ export default async function build(
         : config.experimental.cpus || 4
 
       function createStaticWorker(type: 'app' | 'pages') {
-        const numWorkersPerType = isAppDirEnabled
-          ? Math.max(1, ~~(numWorkers / 2))
-          : numWorkers
-
         let infoPrinted = false
 
         return new Worker(staticWorkerPath, {
@@ -1178,7 +1174,7 @@ export default async function build(
               infoPrinted = true
             }
           },
-          numWorkers: numWorkersPerType,
+          numWorkers,
           forkOptions: {
             env: {
               ...process.env,

test/integration/with-router/test/index.test.js

@@ -7,32 +7,23 @@ import {
   killApp,
   launchApp,
   nextBuild,
-  nextServer,
-  startApp,
-  stopApp,
+  nextStart,
 } from 'next-test-utils'
 import webdriver from 'next-webdriver'
 import { join } from 'path'
 
 describe('withRouter', () => {
   const appDir = join(__dirname, '../')
   let appPort
-  let server
   let app
 
   beforeAll(async () => {
     await nextBuild(appDir)
-    app = nextServer({
-      dir: join(__dirname, '../'),
-      dev: false,
-      quiet: true,
-    })
-
-    server = await startApp(app)
-    appPort = server.address().port
+    appPort = await findPort()
+    app = await nextStart(appDir, appPort)
   })
 
-  afterAll(() => stopApp(server))
+  afterAll(() => killApp(app))
 
   it('allows observation of navigation events using withRouter', async () => {
     const browser = await webdriver(appPort, '/a')