Motivation for this change

Presently, gnupg1compat only works with gnupg22. Without this change, the error

error: attribute 'version' missing, at .../nixpkgs/pkgs/tools/security/gnupg/1compat.nix:4:26

is emitted when evaluating

pkgs.gnupg1compat.override { gnupg = pkgs.gnupg20; }'

Even with that done, 1compat after e34ce9d no longer actually created gpg->gpg2 or gpgv->gpgv2 symlinks at all, making it effectively a noop (and thus ineffective for the gnupg20 package, which -- unlike gnupg22 -- was not modified in the aforementioned commit to create gpg binaries itself).

The symlink-creation behavior is thus reintroduced, but if-and-only-if actually needed.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.