cuirass: Fork inferior processes before creating threads....Works around <https://issues.guix.gnu.org/55441#12>.
Start from commit bd86bbd300474204878e927f6cd3f0defa1662a5,
'open-inferior' uses 'primitive-fork' instead of 'open-pipe*'. As a
result, child process could potentially hang before calling 'execl' due
to undefined behavior when forking a multi-threaded process.
* build-aux/cuirass/evaluate.scm <top level>: Call 'open-inferior'
before 'n-par-for-each'.