Skip child process kills in stack.sh exit trap

Logs are still being cut off in certain circumstances, almost certainly
because the awk process handling the output for verbose logging to a file
dies/is killed early.

Don't kill child processes in the EXIT trap function in this specific case.

Change-Id: I89600b61d59f87e93ff959710d0169e49d2c9845
Merge "Skip child process kills in stack.sh exit trap"