fix hydra build on nixpkgs master

Dependencies

• [2] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• edit in release.nix at line 78
  [2.1705]

  [2.1705]
  + JSON