During build, there's no need to start N workers for type checking and
linting, because there's no concurrency at all for these tasks (both
TypeScript and ESLint are checking all files globally).
During my test of `next build` for a small size project of 5 pages, this
change reduces the peak memory usage by 545MB (as each worker holds a
huge dependency chain). Before (there are too many worker threads
initiated):
<img width="1278" alt="before"
src="https://user-images.githubusercontent.com/3676859/217116925-5594a45c-f1bb-4d11-8699-f6fe12ce7ee5.png">
After:
<img width="1189" alt="CleanShot-2023-02-07-WfQTnB1D@2x"
src="https://user-images.githubusercontent.com/3676859/217116940-a08a256e-b80d-4836-8b97-2837c845435e.png">
It improves the CPU usage as well.
In the future we can spin up multiple workers and assign sub-task for
each, but that only makes sense when the number of pages is large.
NEXT-470
d54d43d4ed
