Fix waiting on child processes blocking indefinitely

[jonatanklosko]

Currently subprocess.run never finishes. Quoting the code comment:

# Some APIs in Python, such as subprocess.run, wait for a child
# OS process to finish. On Unix, this relies on `waitpid` C API,
# which does not work properly if SIGCHLD is ignored, resulting
# in infinite waiting. ERTS ignores the signal by default, so we
# explicitly restore the default handler.

It turns out we do exactly the same in EXLA.