release.nix: Do not rely on currentSystem

Dependencies

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

Change contents

• replacement in release.nix at line 11
  [3.8]→[2.142:170](∅→∅)
  − pkgs = import nixpkgs {};
  [3.8]

  [3.36]
  + pkgs = import nixpkgs { system = "x86_64-linux"; };