Fix a small inbound email glitch

Summary:
Sendmail isn't actually OK with passing ENV stuff via 'aliases', accept it as an
argument instead.

Test Plan:
Sent real email to a real server, got differential updates!

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 233