because this feels still inconsistent:

% nix-env -qaA quodlibet-with-gst-plugins -f dev/nixos/nixpkgs
quodlibet-2.6.3-with-gst-plugins

% nix-env -qa quodlibet-with-gst-plugins -f dev/nixos/nixpkgs 
error: selector `quodlibet-with-gst-plugins' matches no derivations