R1:3844be3f83b5

Minor, fix an MTA issue introduced in D3859.

Prior to D3859, getRequiredLeaseTime() was called before doWork(), which had the critical but subtle side effect of populating `$this->message`. Instead, make the population explicit. This restores email functionality.

Test Plan: ran `phd debug taskmaster` and verified email was delivered

Auditors: btrahan