Previously, in Zulip voyager, the cron jobs would spew error emails
every time they ran, due to this directory not existing.

This also tightens the permissions for the folder and avoids needing
to create a nagios user for Zulip voyager; it should be writeable by
both root and the zulip user and world-readable (and thus readable by
the Nagios user on zulip.com systems).