fix CNA example handling on Windows (#68977)
Azure tests have been failing on CNA examples and I believe it's because
the filter function introduced in #68802 wasn't accounting for Windows
paths when computing the `rootPath`. This normalizes to a POSIX path to
ensure handling is consistent.
[Successful
run](https://dev.azure.com/nextjs/next.js/_build/results?buildId=94852&view=logs&j=14d0eb3f-bc66-5450-3353-28256327ad6c&t=7e988112-57f9-5e14-1c8c-05f6c2b0ea47)
