Add turbo concurrency equal to number of CPUs

[moltar]

https://nodejs.org/api/os.html#os_os_cpus

Can get the value this way:

node -r os -e 'process.stdout.write(os.cpus().length.toString())'

Then include into a step before, and then reference the output in the next step.

[moltar]

Also, asked in the Turbo discord if this should be a built-in feature:

https://discord.com/channels/818588653005176832/818588653147389955/943378730208854086

[moltar]

vercel/turbo#761