Verify canary release

• I verified that the issue exists in Next.js canary release

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #202204271406 SMP PREEMPT Wed Apr 27 16:11:45 UTC 2022
Binaries:
  Node: 16.14.0
  npm: 8.3.1
  Yarn: 1.22.17
  pnpm: 7.0.0
Relevant packages:
  next: 12.1.6
  react: 18.1.0
  react-dom: 18.1.0

What browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

Describe the Bug

next/lint by default does not lint .mjs files (see here), but it is possible to pass --ext .mjs.

when next build runs the linter, it uses the same default file extensions (see here), however there is no way to provide custom file extensions like with the --ext CLI option.

Expected Behavior

see above

To Reproduce

1. git clone git@github.com:stefanprobst/issue-next-lint-mjs.git
2. yarn run lint shows no error
3. yarn run lint:ext shows error because it passes --ext .mjs
4. yarn run build shows no error