Make NIX_REMOTE_SYSTEMS configurable

Dependencies

• [2] TTZ26BJQ Unify Hydra's NixOS module with the one used for hydra.nixos.org
• [3] KKHVM4KA Move GC roots to /nix/var/nix/gcroots/hydra
• [*] D3MDJONY
• [*] G2RULGMM hydra-module: add config.extraEnv
• [*] ZYEFL36Q Ported/merged properties from the TUD configuration's hydra-module

Change contents

• edit in hydra-module.nix at line 23
  [2.182]

  [6.0]
  + NIX_REMOTE_SYSTEMS = concatStringsSep ":" cfg.buildMachinesFiles;
• edit in hydra-module.nix at line 153
  [3.192]

  [5.2521]
  +

  + buildMachinesFiles = mkOption {

  + type = types.list types.path;

  + default = [];

  + example = [ "/etc/nix/machines" "/var/lib/hydra/provisioner/machines" ];

  + description = "List of files containing build machines.";

  + };
• edit in hydra-module.nix at line 322
  [7.2337]

  [7.2337]
  + WorkingDirectory = baseDir;