Add updateEnv() to ExecFuture for adjusting environmental variables

Summary: Ref T4195. I have a use case where I need to tweak one variable that something else has already set with setEnv().

Test Plan: Used this in next diff.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7762