Update build with step states during build execution

[imjasonh]

Expected Behavior

As a build executes on-cluster, watch kubectl get build foo -oyaml should show the current state of completed/ongoing steps.

Actual Behavior

.Spec.Status.StepStates is only updated upon completion of the pod executing the build.

The on-cluster builder implementation uses registerDoneCallback to receive notifications about pod state when the pod completes, but it should also get updates during execution to update its stepStates.

Steps to Reproduce the Problem

1. Create a build with multiple steps, each of which takes some amount of time (e.g., steps that each sleep 10)
2. kubectl apply -f the-build.yaml
3. watch kubectl get build the-build -oyaml
4. The build's status is not updated until the build completes (successfully or otherwise)

[shashwathi]

/assign @shashwathi