sg: `sg setup` deadlocks on Arch Linux

Created by: LawnGnome

• Platform information: Arch Linux

Steps to reproduce:

1. Run sg setup (or sg setup -c).

Expected behavior:

No hang.

Actual behavior:

sg hangs after displaying the initial progress tree.

This appears to be some sort of deadlock in the check concurrency — manually patching the runner concurrency to be 1 instead of 10 allows it to proceed as expected. An obvious workaround for now would be to drop the default concurrency on Linux, but I would like to get some input and understand what's causing the deadlock a little better before doing that.