hydra-module: add default to buildMachinesFiles

nixos/modules/services/continuous-integration/hydra/default.nix

···
 
       buildMachinesFiles = mkOption {
         type = types.listOf types.path;
-        default = [];
+        default = [ "/etc/nix/machines" ];
         example = [ "/etc/nix/machines" "/var/lib/hydra/provisioner/machines" ];
         description = "List of files containing build machines.";
       };