Make all build steps support variables

Summary: Ref T1049. This generally simplifies things. The steps which don't support variables generally don't make sense to support varaibles anyway.

Test Plan: Edited some steps.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D8588