Fix API route matched as page on Windows in dev (#46238)

Use `path.join` to build the path instead of a template string. This ensures the format is correct on all platforms. Currently it incorrectly matches API routes as pages due to forward slashes in `/api/` being added to the end, which doesn't work on Windows.

This caused the dev server to incorrectly log `Duplicate page detected`.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/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`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

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

packages/next/src/server/future/route-matcher-providers/dev/dev-pages-route-matcher-provider.ts

@@ -58,7 +58,7 @@ export class DevPagesRouteMatcherProvider extends FileCacheRouteMatcherProvider<
     // so we need to include the pages directory.
 
     // TODO: could path separator normalization be needed here?
-    if (filename.startsWith(`${this.pagesDir}/api/`)) return false
+    if (filename.startsWith(path.join(this.pagesDir, '/api/'))) return false
 
     for (const extension of this.extensions) {
       // We can also match if we have `pages/api.${extension}`, so check to