Function waitForPostRunHookAndExit

Wait for the postrun hook to finish (so auto-upgrade has a chance to run) and then tree-kill the current process tree before exiting.

Long-running interactive commands like app dev need this when the user terminates the command via q or Ctrl+C. The dev sub-processes such as servers and watchers keep the event loop alive, so even after oclif's postrun hook completes the node process won't exit on its own and we have to treeKill the process tree. We must not do that before the postrun hook has actually finished running auto-upgrade, otherwise we would kill the upgrade mid-way while npm install is still running.

The flag postRunHookCompleted is flipped at the very end of the hook after autoUpgradeIfNeeded resolves, so polling it here is safe.