BTW to set environment variables in home manager you can do: 
```
  systemd.user.sessionVariables = {
    # firefox screencapture
    MOZ_ENABLE_WAYLAND="1";
    MOZ_USE_XINPUT2="1";

    # SDL
    SDL_VIDEODRIVER="wayland";

    # QT
    QT_QPA_PLATFORM="wayland";
  };
```