Revert concurrent yarn runs

Created by: eseliger

They aren't thread safe and for up-to-date installs it's equally fast

some quick time measurements show it doesn't matter when the packages are already up-to-date.

With mutex
5.28
5.33
4.99
avg = 5.2

Without mutex
4.95
5.36
5.36
avg = 5.22

Serial
4.96
5.21
5.09
avg = 5.09