Make WorkingCopy build step slightly more durable

Summary: Fixes T9631. Build steps created before I added this option may not have it specified, which could throw later. Make handling a little more robust.

Test Plan: Will ask @yelirekim to report back.

Reviewers: chad

Reviewed By: chad

Subscribers: yelirekim

Maniphest Tasks: T9631

Differential Revision: https://secure.phabricator.com/D14336