Include proper instrumentation entry in required files manifest (#68983)
### What
When the `edge-instrumentation.js` is not built, we do not include it
into the serve required manifest.
We moved the write of required server files manifest to a late phase,
where we used the edge routes counter to determine if we need to write
the edge entry file of instrumentation into manifest
x-ref:
https://github.com/vercel/next.js/actions/runs/10416595866/job/28850695138