A child process (e.g. CVC4) might catch SIGXCPU. If it gets stuck then, it
won't consume any additional cpu time, so the system won't forcibly kill
it. So why3-cpulimit has to kill it.

Note that, if the system is overloaded, why3-cpulimit might kill the child
process before it has even reached its cpu time limit. Hopefully, the 60'
additional time will suffice in practice.