This is necessary for the toolbox script to work inside the toolbox
container [1]. Having the flatpak-spawn command, provided by the
flatpak-xdg-utils package, readily available in the base image makes
for a better out of the box experience.
The flatpak-xdg-utils package is not available on Fedora 28. Given
that Fedora 28 is getting close to the end of its life cycle, it's
probably not worth doing all the paperwork to offer the package there.
[1] Commit 5b3d234c9ehttps://github.com/debarshiray/toolbox/pull/54
