Handle derivations with system type 'builtin'

Dependencies

• [2] 4ZJIJCVF Slight cleanup
• [3] HJOEIMLR Refactor

Change contents

• replacement in src/hydra-queue-runner/state.hh at line 254
  [2.86]→[2.86:152](∅→∅)
  − if (!systemTypes.count(step->drv.platform)) return false;
  [2.86]

  [2.152]
  + if (!systemTypes.count(step->drv.platform == "builtin" ? nix::settings.thisSystem : step->drv.platform))

  + return false;