Don't try to create the Aphlict log directory if it already exists

Summary: Ref T6944. This was not quite implemented correctly in D11387.

Test Plan: Saw no more exceptions about being unable to create /var/log.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: maxhodak, Korvin, epriestley

Maniphest Tasks: T6944

Differential Revision: https://secure.phabricator.com/D11397