Move build workers count change behind flag (#49937)
It seems the build workers count calc change that was made in https://github.com/vercel/next.js/pull/47823 caused a regression in performance as we aren't utilizing as many workers as we previously did so this moves that change behind an experimental flag while additional tweaking is investigated.
x-ref: https://github.com/vercel/next.js/pull/47823
Fixes: https://github.com/vercel/next.js/issues/49593
