Describe the bug
gnuradio-companion does not work; it looks like it cannot find some Python modules / the PYTHONPATH is incomplete.

To Reproduce
Steps to reproduce the behavior:

1. Install gnuradio: nix-env -iA nixos.gnuradio-with-packages or gnuradio-companion
2. Try to start gnuradio-companion:

   $ gnuradio-companion
   ImportError
   
   Failed to initialize GTK. If you are running over ssh, did you enable X forwarding and start ssh with -X?
   
   (No module named gtk)
   

Metadata

• system: "x86_64-linux"
• host os: Linux 5.4.39, NixOS, 20.03.1795.d6c1b566b77 (Markhor)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.4
• channels(root): "nixos-20.03.1795.d6c1b566b77"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos