hydra-module: fix useSubstitutes configuration option

Dependencies

• [2] G7LZJ4VA hydra-module.nix: add 'useSubstitutes' option to control use of binary caches
• [3] 4HDPYRJ6 Update the hydra-queue-runner unit
• [4] TTZ26BJQ Unify Hydra's NixOS module with the one used for hydra.nixos.org
• [5] 2ANV36PB Hackery
• [6] 7XOH2DGH hydra-module.nix: Give types to options
• [7] 36YF4WOZ Remove unnecessary call to hydra-queue-runner --unlock
• [8] ZYEFL36Q Ported/merged properties from the TUD configuration's hydra-module
• [*] D3MDJONY

Change contents

• replacement in hydra-module.nix at line 327
  [3.1684]→[2.644:828](∅→∅)
  − { ExecStart = "@${cfg.package}/bin/hydra-queue-runner hydra-queue-runner -v --option build-use-substitutes ${if optionalString cfg.useSubstitutes then "true" else "false"}";
  [3.1684]

  [3.85]
  + { ExecStart = "@${cfg.package}/bin/hydra-queue-runner hydra-queue-runner -v --option build-use-substitutes ${if cfg.useSubstitutes then "true" else "false"}";