Filter out StepStates for implicit steps

[imjasonh]

Expected Behavior

When I run a build that has N steps, I see N StepStates, one for each step I requested.

Actual Behavior

I see N+1 StepStates, or N+2 if my build specified source and had to be fetched.

When populating StepStates, we should filter out the first one, which is the build running creds-init, and if the build specifies source, we should filter out the second one as well.

If we add other implicit steps (e.g., build output collection), we should make sure to filter those out too.