Fix conflict between defaultPathMap and default 404.html export (#8366)

* Fix conflict between defaultPathMap and default 404.html export，if have a page in pages/404.js

* Update packages/next/export/index.js
Co-Authored-By: NJJ Kasper <jj@jjsweb.site>

* add test export-override-404

packages/next/export/index.js

@@ -140,7 +140,9 @@ export default async function (dir, options, configuration) {
     distDir,
     buildId
   })
-  exportPathMap['/404.html'] = exportPathMap['/404.html'] || { page: '/_error' }
+  if (!exportPathMap['/404']) {
+    exportPathMap['/404.html'] = exportPathMap['/404.html'] || { page: '/_error' }
+  }
   const exportPaths = Object.keys(exportPathMap)
   const filteredPaths = exportPaths.filter(
     // Remove API routes

test/integration/export-override-404/pages/404.js

+export default () => <p>this is a 404 page override the default 404.html</p>

test/integration/export-override-404/test/index.test.js

+/* eslint-env jest */
+/* global jasmine */
+import fs from 'fs'
+import { join } from 'path'
+import { promisify } from 'util'
+import { nextBuild, nextExport } from 'next-test-utils'
+
+jasmine.DEFAULT_TIMEOUT_INTERVAL = 1000 * 60 * 5
+const readFile = promisify(fs.readFile)
+const appDir = join(__dirname, '../')
+const outdir = join(appDir, 'out')
+
+describe('Export with a page named 404.js', () => {
+  beforeAll(async () => {
+    await nextBuild(appDir)
+    await nextExport(appDir, { outdir })
+  })
+
+  it('should export a custom 404.html instead of default 404.html', async () => {
+    const html = await readFile(join(outdir, '404.html'), 'utf8')
+    expect(html).toMatch(/this is a 404 page override the default 404\.html/)
+  })
+})