nixos/gnupg: define agent.pinentryFlavor statically

We don't want to depend on the *enabled* flavors of pinentry but on all
*possible* ones.

nixos/modules/programs/gnupg.nix

@@ -64,7 +64,8 @@ in
     };
 
     agent.pinentryFlavor = mkOption {
-      type = types.nullOr (types.enum pkgs.pinentry.flavors);
+      # keep in sync with all possible favlors of pinentry
+      type = types.nullOr (types.enum [ "curses" "tty" "emacs" "gtk2" "gnome3" "qt" ]);
       example = "gnome3";
       default = defaultPinentryFlavor;
       defaultText = literalMD ''matching the configured desktop environment'';
@@ -232,7 +233,4 @@ in
       }
     ];
   };
-
-  # uses attributes of the linked package
-  meta.buildDocsInSandbox = false;
 }

pkgs/tools/security/pinentry/default.nix

@@ -22,6 +22,7 @@ let
       lib.optionalString (flag != null)
         (lib.enableFeature (lib.elem f enabledFlavors) ("pinentry-" + flag));
 
+  # keep in sync with programs.gnupg.agent.pinentryFlavor
   flavorInfo = {
     curses = { bin = "curses"; flag = "curses"; buildInputs = [ ncurses ]; };
     tty = { bin = "tty"; flag = "tty"; };