When the sandbox runs in attached more, e.g. runsc do, runsc run, the
sandbox lifetime is controlled by the parent process. This wasn't working
in all cases because PR_GET_PDEATHSIG doesn't propagate through execve
when the process changes uid/gid. So it was getting dropped when the
sandbox execve's to change to user nobody.
PiperOrigin-RevId: 300601247