Don't create a propagated build step to the same build

Dependencies

• [2] HUUZFPPK Fix race between the queue monitor and the builder threads
• [3] E7WP35SF Create build step for non-top-level cached failures
• [4] O7MWBXF6 Doh
• [*] 24BMQDZA Start of single-process hydra-queue-runner

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 1256
  [2.8106]→[2.8106:8176](∅→∅)
  − buildStepStatus, result.errorMsg, build->id);
  [2.8106]

  [2.8176]
  + buildStepStatus, result.errorMsg, build == build2 ? 0 : build->id);