release.nix: add missing dependency on nix

Dependencies

• [2] UQYHPQ6U Run PostgreSQL during "make check"
• [3] X7IAG7JT Simplify running nix-shell
• [4] L7RW467Z Add a test for darcs inputs.
• [5] 2554VN4T Fix ‘Unable to handle files with the extension 'conf'’
• [6] 6K5PBUUN Use buildEnv to combine Hydra's Perl dependencies
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 121
  [3.1396]→[3.78:102](∅→∅)
  − perlDeps perl
  [3.1396]

  [2.0]
  + perlDeps perl nix