Limit build workers to pages+1 (#90475)
During build time, we spawn a pool of workers for static page
generation. The size is `max(4, NUM_CORES - 1)`.
With smaller sites, this ends up with far more workers than pages. This
tries to cap the number of workers we actually spin up to something more
appropriate for the number of pages. This change saved 90ms on an app
with fewer static pages, on a 10-core Macbook.
After this change, the behavior will be:
- If you have 16 cores and 4 pages, we start 5 workers
- If you have 8 cores and 100 pages, we start 7 workers
