Kill the child processes spawned by a run command

When the timeout occurs it is important to ensure clean up of child
processes. Killing only the direct process created by a command can
leave child processes running.

For example a pmbootstrap.py install will run apk add. This run command
creates multiple processes as follows:
(cmd line arguments snipped for readability)

  $ ps -e -o pid,ppid,pgid,cmd
  PID  PPID  PGID CMD
  31738 23247 31738 python3 ./pmbootstrap.py -t 15 install --no-fde
  31746 31738 31738 sudo env -i /bin/sh -c ... ;apk --no-progress add
  31747 31746 31738 /bin/sh -c ... ;apk --no-progress add
  31748 31747 31738 apk --no-progress add

The root process of the run command is PID 31746. We want to kill
the child processes too. Otherwise only running kill -9 31746 will leave
the processes 31747 and 31748 running.