[build] Add ocunit-config options for xcodebuild

Summary:
If you don't specify a workspace, xcodebuild assumes you're building
the project, which isn't the case when you're using Cocoapods. This results
in the wrong derived data directory being specified.

When you do specify a workspace, and that workspace has multiple build targets
from different projects, xcodebuild will spit out multiple values for
PROJECT_TEMP_DIR. To get around this, we can specify which project build name
we should use to determine PROJECT_TEMP_DIR.

Test Plan:
Run `arc unit` in a project with a workspace and multiple projects
and build targets. See that ocunit can successfully find the build output.

Reviewers: bobo

Reviewed By: bobo

Differential Revision: https://phabricator.robinhood.com/D592