fix: add locale prefix to middleware preflight url (#35911)
Fixes part of https://github.com/vercel/next.js/issues/34274
Navigating to `/` causes to redirect preflight request to a url of browser language like `/en`.
This PR fixes to add the locale prefix always so that the redirect does not happen anymore and middleware can get a correct locale.
## 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 `yarn lint`
