fix: don't treat output: "standalone" as custom server (#38831)

Fixes #38742

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm lint`
- [ ] The examples guidelines are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing.md#adding-examples)

packages/next/build/utils.ts

@@ -1230,6 +1230,7 @@ server.listen(currentPort, (err) => {
     port: currentPort,
     dir: path.join(__dirname),
     dev: false,
+    customServer: false,
     conf: ${JSON.stringify({
       ...serverConfig,
       distDir: `./${path.relative(dir, distDir)}`,