Build with turbopack does not opt out robots.txt and sitemap.xml from static generation

[sdoomz]

Link to the code that reproduces this issue

https://github.com/sdoomz/nextjs-turbopack-robots-cache

To Reproduce

1. npm install
2. npm run build
3. npm run start
4. Open http://localhost:3000/sitemap.xml and http://localhost:3000/robots.txt -
5. Try to spot https://next.com/ on the opened page, in case you see https://localhost:3000/ - the bug is there.

Current vs. Expected behavior

next build --turbopack doesn't opt out robots.txt and sitemap.xml from static generation. Running next build without --turbopack works as expected.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.5.0: Tue Apr 22 19:53:27 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6041
  Available memory (MB): 49152
  Available CPU cores: 14
Binaries:
  Node: 23.11.0
  npm: 10.9.2
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.4.0-canary.48 // Latest available version is detected (15.4.0-canary.48).
  eslint-config-next: 15.1.8
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack

Which stage(s) are affected? (Select all that apply)

next build (local)

Additional context

No response