Add support for specifying working directory in lego-determinator (fa…

…cebook#7901)

Summary:
So we could change the repo or repo structure.

Pull Request resolved: facebook#7901

Test Plan:
works with existing job: https://www.internalfb.com/intern/sandcastle/job/13510799254899745/
works with internal repo: https://www.internalfb.com/intern/sandcastle/job/27021598137143731

Reviewed By: pdillinger

Differential Revision: D26108239

Pulled By: jay-zhuang

fbshipit-source-id: ac24559653d9abc012068e8accd7606505bd5b88