Propagate PHABRICATOR_ENV into VCS commands explicitly

Summary: Fixes T4155. See discussion in T4155.

Test Plan: @mbishopim3 confirmed this fixes his issue.

Reviewers: btrahan, chad

Reviewed By: chad

CC: mbishopim3, aran

Maniphest Tasks: T4155

Differential Revision: https://secure.phabricator.com/D7646