Re-add internal env for providing page paths (#70235)
This re-adds internal env variables to allow providing page paths
directly for debugging purposes. This is NOT a public API and should not
be leveraged outside of internal Next.js debugging.
x-ref: https://github.com/vercel/next.js/pull/67134
x-ref: [slack
thread](https://vercel.slack.com/archives/C04DUD7EB1B/p1726519130567199)
