Use nixUnstable

Dependencies

• [2] NRNR4QVJ Use stable Nix
• [3] T4LLYESZ * Nix expression for building Hydra.
• [4] 6K5PBUUN Use buildEnv to combine Hydra's Perl dependencies

Change contents

• replacement in release.nix at line 79
  [3.10]→[2.82:108](∅→∅)
  − #nix = nixUnstable;
  [3.10]

  [3.874]
  + nix = nixUnstable;