Fix instrumentation with only pages (#62622)
This ensures we properly transpile `instrumentation.ts` when only
`pages` is being used as previously we were relying on the `app`
specific loaders which aren't configured when an `app` directory isn't
present. We regressed on this in
https://github.com/vercel/next.js/pull/60984 as it was working as
expected prior to this commit
x-ref: [slack
thread](https://vercel.slack.com/archives/C011GK1JUBA/p1709075846401649?thread_ts=1706643408.233909&cid=C011GK1JUBA)
Closes NEXT-2632
