Build: Log amount of workers during static generation (#85706)
## What?
Added the amount of workers used to the relevant steps as `using X
workers`.
Example:
```
✓ Compiled successfully in 610.3ms
✓ Finished TypeScript in 954.4ms
✓ Collecting page data using 15 workers in 207.5ms
✓ Generating static pages using 15 workers (4/4) in 258.4ms
✓ Finalizing page optimization in 6.7ms
```
