Allow cancelling builds marked as busy

Dependencies

• [2] UOINKJ2J Add an action to cancel all builds in a jobset eval
• [3] GEADFVZ5 hydra-queue-runner: Improved scheduling
• [4] 2IQRXLWE Support cancelling builds
• [*] 2GK5DOU7 * Downloading closures.

Change contents

• replacement in src/lib/Hydra/Helper/Nix.pm at line 427
  [2.1265]→[2.1265:1330](∅→∅)
  − $builds = $builds->search({ finished => 0, busy => 0 });
  [2.1265]

  [2.1330]
  + $builds = $builds->search({ finished => 0 });