Fix errors that sometimes occur on hs_exit

[facundominguez]

Calling hs_exit() sometimes produces errors in CI like:

�^ �C�: newBoundTask: RTS is not initialised; call hs_init() first
���� Error: The command exited with status 1

I've tried a few things in the past, but I haven't managed to get rid of it, and I'm unsure if there is a way to fix it. This is probably related to the fact that the failing programs are using bound threads.

A first step would be creating a minimal example to reproduce this failure. Then we could submit a bug to GHC devs.