nixos/greetd: autostart GNOME Keyring when enabled

The PAM service `greetd` creates now autostarts GNOME Keyring when it is
enabled via `services.gnome.gnome-keyring.enable`.

Should cause a lot fewer headaches for people like me who didn't know
why GNOME Keyring was complaining that it was never supplied with login details.

Fixes #246197

nixos/modules/services/display-managers/greetd.nix

···
     security.pam.services.greetd = {
       allowNullPassword = true;
       startSession = true;
+      enableGnomeKeyring = mkDefault config.services.gnome.gnome-keyring.enable;
     };
 
     # This prevents nixos-rebuild from killing greetd by activating getty again