Instead of just going for "whatever is the oldest build we know of",
use the following first:
- Is the step more constrained? If so, schedule it first to avoid
filling up "more desirable" build slots with less constrained builds.
- Does the step have more dependents? If so, schedule it first to try
and maximize open parallelism and breadth of scheduling options.
(cherry picked from commit b8d03adaf4)
720db63d52