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