Fix building on Nixpkgs 17.09

Dependencies

• [2] GUOCMPZ2 Use Nix 2.0 and default GCC
• [3] ICYJNJWH Revert "Move build logic for the core hydra package into its own default.nix."
• [4] UBPWXGKF Move build logic for the core hydra package into its own default.nix.
• [5] 6K5PBUUN Use buildEnv to combine Hydra's Perl dependencies
• [6] GGWH5GY7 Remove unused dependency on aws-sdk-cpp
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 44
  [3.1188]→[2.0:26](∅→∅)
  − #nix = nixUnstable;
  [3.1188]

  [3.731]
  + nix = pkgs.nixStable2 or pkgs.nix;