Fix Linux build on Darwin

Dependencies

• [2] QPDG4NEU release.nix: Use fetchGit to get Nixpkgs
• [3] IZYR3KTL release.nix: Get rid of the "system" argument
• [4] YQWH4POV * Simplify.
• [5] T4LLYESZ * Nix expression for building Hydra.
• [6] 6CZHII3Q Use <...> syntax

Change contents

• edit in release.nix at line 40
  [3.143]

  [3.737]
  + let pkgs = import nixpkgs { inherit system; }; in
• replacement in release.nix at line 42
  [3.738]→[2.171:216](∅→∅)
  − with import nixpkgs { inherit system; };
  [3.738]

  [3.738]
  + with pkgs;