hydra-queue-runner: Recycle finishedDrvs

Dependencies

• [2] MHVIT4JY Split hydra-queue-runner.cc more
• [3] VQISTKOP hydra-queue-runner: Use substitutes
• [4] 73YR46NJ hydra-queue-runner: Write directly to a binary cache
• [5] YFD6CBV3 hydra-queue-runner: Fix handling of missing derivations

Change contents

• edit in src/hydra-queue-runner/queue-monitor.cc at line 117
  [2.22981]

  [2.22981]
  + std::set<Path> finishedDrvs;
• edit in src/hydra-queue-runner/queue-monitor.cc at line 143
  [2.23973]→[2.23973:24028](∅→∅)
  − std::set<Path> finishedDrvs; // FIXME: re-use?