Prefer GnuPG 2.0.x by default.

"nix-env -iA gnupg" installs the 2.0.x version of GNU Privacy Guard. This patch
ensures that "nix-env -i gnupg" chooses the same version, instead of installing
GnuPG 2.1.x, which is considered a "development version".

Closes https://github.com/NixOS/nixpkgs/issues/11899.