[build-sourcemaps] Allow inspecting prerender worker (#79098)
If `--inspect` or `--inspect-brk` is specified, prerender will no longer time out since that's most likely not wanted (e.g. you'd always get timeouts if you set breakpoints).
Inspecting prerender workers is best done with `experimental.cpus: 1`.
Only the first prerender worker can be inspected since we can't set `NODE_OPTIONS` based off of the worker index.
This PR also sets up infra for inspecting arbitrary workers. Their debug port would need to be coordinated within `getNextBuildDebuggerPortOffset`. This doesn't work if worker creation is shared between `next build` and `next dev` which I haven't checked if that's the case.
