Create build step for non-top-level cached failures

Dependencies

• [2] HUUZFPPK Fix race between the queue monitor and the builder threads
• [3] HPJKBFZ4 Handle concurrent finishing of the same build
• [4] OCZ4LSGG Automatically retry aborted builds
• [*] 24BMQDZA Start of single-process hydra-queue-runner

Change contents

• replacement in src/hydra-queue-runner/hydra-queue-runner.cc at line 1227
  [2.7894]→[2.7894:8030](∅→∅)
  − if (build == build2) continue;

  − if (cachedFailure && build2->drvPath == step->drvPath) continue;
  [2.7894]

  [2.8030]
  + if ((cachedFailure && build2->drvPath == step->drvPath) ||

  + (!cachedFailure && build == build2))

  + continue;