Use default concurrency value for lerna run

[dlarocque]

The default concurrency value for lerna run is the number of logical CPU cores. This is better than arbitrarily using 4 threads, since different machines may be able to handle more.

The tests were able to run faster on my machine after this, since it has 8 logical cpu cores (os.cpus().length is 8)

Related:

• https://fig.io/manual/lerna/run
• https://github.com/lerna/lerna/pull/1931