gnome3.gcr: prevent dependency cycle

When overriding gnupg to uss pinentry gnome3 frontend, there is
a dependency cycle:

    gnupg → pinentry_gnome → gcr → gnupg

This commit overrides the gnupg required by gcr to not build GUI.

pkgs/desktops/gnome-3/core/gcr/default.nix

@@ -7,8 +7,10 @@ stdenv.mkDerivation rec {
 
   outputs = [ "out" "dev" ];
 
-  buildInputs = [
-    pkgconfig intltool gnupg gobjectIntrospection libxslt
+  buildInputs = let
+    gpg = gnupg.override { guiSupport = false; }; # prevent build cycle with pinentry_gnome
+  in [
+    pkgconfig intltool gpg gobjectIntrospection libxslt
     libgcrypt libtasn1 dbus_glib pango gdk_pixbuf atk makeWrapper vala_0_32
   ];