heavy_compute bench has bad parameters

[hymm]

bevy 0.8-dev e5eb837

What happened

The current heavy_compute benchmark is testing 1000 entities / 128 batch = 8 batches and using the default task pool. This makes the test overly sensitive to the number of cores on a machine.

Desired Fix

Not sure what the best fix is here. Playing with the batch size, I was seeing the best perf at around 8 to 16. We could change the batch size or increase the number of entities or fix the number of threads to a lower number. Or vary the number of threads, Or vary the batch size.

[mockersf]

#4777 would remove the need to specify the batch size manually