gnupg: disable gui/pinentry support by default

This solves the dependency cycle in gcr alternatively so there won't be
two gnupg store paths in a standard NixOS system which has udisks2 enabled
by default.

NixOS users are expected to use the gpg-agent user service to pull in the
appropriate pinentry flavour or install it on their systemPackages and set
it in their local gnupg agent config instead.

Co-authored-by: Florian Klink <flokli@flokli.de>