Support custom servers with minimal node modules (#72966)
## Fixing a bug
### Related issues
fixes #64031
### Why is this change needed
Per the Next.js
[docs](https://nextjs.org/docs/pages/api-reference/next-config-js/output)
> Next.js' production server is also traced for its needed files and
output at .next/next-server.js.nft.json which can be leveraged in
production.
When I tried doing this my [custom
server](https://nextjs.org/docs/pages/building-your-application/configuring/custom-server)
would crash on startup because of this codepath
https://github.com/vercel/next.js/blob/canary/packages/next/src/server/config.ts#L1022
My first attempt at fixing this involved changing
```
if (!process.env.__NEXT_PRIVATE_STANDALONE_CONFIG) {
```
to
```
if (phase === PHASE_PRODUCTION_SERVER) {
```
but this is not guaranteed to work because the next.config.js and it's
required dependencies may not be present causing the server to use the
default config
The best solution is to mimic standalone mode as closely as possible
### What does this change do
This change allows a custom server running in production with only
required files to pass in a preloaded next config (most likely from
`required-server-files.json`)
This is very similar to how the [standalone server
works](https://github.com/vercel/next.js/blob/bc49287063d6afd50a7361566b14f1ab6c26b136/packages/next/src/build/utils.ts#L2142)
### Testing
I'd like to get an initial round of feedback from the next team before I
dive into writing an integration test. I patched nextjs in my repo and
this solution worked
