Give services.hydra.package a reasonable default

Dependencies

• [2] PMDWFZHW hydra-module.nix: Fix setting the logo
• [*] D3MDJONY

Change contents

• edit in hydra-module.nix at line 146
  [4.2592]

  [2.79]
  +

  + services.hydra.package = mkDefault ((import ./release.nix {}).build.x86_64-linux);