`exec.CombinedOutput()` intermittently returns `ECHILD` due to our
signal handling.
`wait(2)`: https://man7.org/linux/man-pages/man2/wait.2.html
> ECHILD (for waitpid() or waitid()) The process specified by pid
> (waitpid()) or idtype and id (waitid()) does not exist or is
> not a child of the calling process. (This can happen for
> one's own child if the action for SIGCHLD is set to SIG_IGN.
> See also the Linux Notes section about threads.)
Fix#4387
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
403dc83a29