R1:5f774f7008b4

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…