Fix perl search path; avoid reference to build path (#471)

Dependencies

• [2] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [3] IZYR3KTL release.nix: Get rid of the "system" argument
• [4] GIBXI3X2 preHook -> shellHook
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 151
  [2.3339]→[2.3339:3382](∅→∅)
  − PERL5LIB=$(pwd)/src/lib:$PERL5LIB;
  [2.3339]

  [2.3382]
  + ${lib.optionalString shell "PERL5LIB=$(pwd)/src/lib:$PERL5LIB"}