Trying to run sandboxed GStreamer applications from within jhbuild, for
example, would make those applications fail to find their plugins.
$ LANG=C flatpak run org.gnome.Totem.Devel
** (totem:2): WARNING **: 19:32:06.406: Element 'gtkglsink' is missing, verify your installation
** (totem:2): WARNING **: 19:32:06.406: Element 'glsinkbin' is missing, verify your installation
Don't propagate those GStreamer environment variables to within the
sandbox to avoid that problem.
See https://gitlab.gnome.org/GNOME/totem/-/issues/504