Implement `setEnv`.

Summary: Implements `setEnv` on `ExecFuture` so that the environment variable can be set.

Test Plan: Completely untested.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7396