Set the working directory when providing SSH / SFTP interfaces

Summary: Ref T1049. Set the working directory when executing commands on Drydock hosts. Without this set, they execute in the user's default home directory.

Test Plan: Ran a build and saw the correct working directory when running pwd.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: CanadianBadass, epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D10293