update number of workers to a decent default

[0xalpharush]

Describe the desired feature

The current default is one but we should try to use all available CPU by default to maximize performance

[ggrieco-tob]

This was upgraded to 4 (if cores are available). We can increase it, but using all the available CPU means that the user experience can be degraded if the developer is using short fuzzing campaign while they are developing new code.