nixos/xfce: don't set a bloated GST_PLUGIN_PATH
nixos/environment: do not set GST_PLUGIN_SYSTEM_PATH
xfce4-mixer: use wrapper to set GST_PLUGIN_SYSTEM_PATH
Merge pull request #3653 from iyzsong/nixos/xfce4-mixer

nixos: don't set variables for gstreamer-0.10 (fix #3652)