Fix React channel env missing in custom server (#49168)

Fixes #48948
([repro](#48948 (comment))).
When running inside a custom server with app dir, we should always opt
into the prebundled React with correct channels.

Thanks @karlhorky for help testing it!

Fixes #49169 too.

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/next/src/build/index.ts

@@ -285,8 +285,6 @@ export default async function build(
       const isAppDirEnabled = !!config.experimental.appDir
 
       if (isAppDirEnabled) {
-        process.env.NEXT_PREBUNDLED_REACT = '1'
-
         if (!process.env.__NEXT_TEST_MODE && ciEnvironment.hasNextSupport) {
           const requireHook = require.resolve('../server/require-hook')
           const contents = await promises.readFile(requireHook, 'utf8')

packages/next/src/server/next.ts

@@ -178,9 +178,18 @@ export class NextServer {
             )
           }
         }
-        if (conf.experimental.appDir) {
-          process.env.NEXT_PREBUNDLED_REACT = '1'
+
+        if (this.options.customServer !== false) {
+          // When running as a custom server with app dir, we must set this env
+          // to correctly alias the React versions.
+          if (conf.experimental.appDir) {
+            process.env.__NEXT_PRIVATE_PREBUNDLED_REACT = conf.experimental
+              .serverActions
+              ? 'experimental'
+              : 'next'
+          }
         }
+
         this.server = await this.createServer({
           ...this.options,
           conf,

test/e2e/app-dir/navigation/navigation.test.ts

@@ -7,7 +7,7 @@ createNextDescribe(
   {
     files: __dirname,
   },
-  ({ next, isNextDev, isNextDeploy }) => {
+  ({ next, isNextDeploy }) => {
     describe('query string', () => {
       it('should set query correctly', async () => {
         const browser = await webdriver(next.url, '/')
@@ -184,9 +184,7 @@ createNextDescribe(
           }
 
           expect(stored).toEqual({
-            // Not actually sure why this is '2' in dev. Possibly something
-            // related to an update triggered by <HotReload>?
-            'navigate-https://example.vercel.sh/': isNextDev ? '2' : '1',
+            'navigate-https://example.vercel.sh/': '1',
             'navigation-supported': 'true',
           })
         })