gnupg20: set version variable to fix gnupg1compat

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; }'
```