Set `isSystemUser = true;` for each statically declared user

Dependencies

• [2] 2ANV36PB Hackery
• [*] D3MDJONY
• [*] TTZ26BJQ Unify Hydra's NixOS module with the one used for hydra.nixos.org

Change contents

• edit in hydra-module.nix at line 207
  [5.807]

  [5.807]
  + isSystemUser = true;
• edit in hydra-module.nix at line 215
  [5.992]

  [2.52]
  + isSystemUser = true;
• edit in hydra-module.nix at line 222
  [5.1102]

  [5.1102]
  + isSystemUser = true;