box: fix high CPU usage while on_shutdown triggers are running

Currently this script causes 100% CPU usage for 10 sec, because
os.exit() infinitely yields to the scheduler until on_shutdown
fiber completes and breaks the event loop. Fix this by a sleep.

```
box.ctl.set_on_shutdown_timeout(100)
box.ctl.on_shutdown(function() require('fiber').sleep(10) end)
os.exit()
```

Closes #6801

NO_DOC=bugfix
NO_TEST=don't know how to catch this by a test

Co-authored-by: Georgy Moshkin <louielouie314@gmail.com>