Stop build target start times from being overwritten on reentry
Summary:
See PHI615. Ref T13130. An install is reporting that "Lease Working Copy" build steps always report "Built instantly" after completion.
I'm not 100% sure that this is the fix, but I'm like 99% sure: "Lease Working Copy" build steps yield after they ask Drydock for a lease. They will later reenter `doWork()`, see that the lease is filled, and complete.
Right now, we reset the start time every time we enter `doWork()`. Instead, set it only if it hasn't been set yet.
Test Plan: This is low-risk and a bit tricky to reproduce locally, but I'll run some production builds and see what they look like.
Reviewers: amckinley
Reviewed By: amckinley
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13130
Differential Revision: https://secure.phabricator.com/D19412