bug#44997: Flatpaks are invisible to application launchers

[guixuser6392 via Bug reports for GNU Guix]

Applications installed by Flatpak can't be discovered by application launchers. 
A fix to this is to add the directories '/var/lib/flatpak/exports/share' and 
'$HOME/.local/share/flatpak/exports/share' to the search path set by the 
XDG_DATA_DIRS environment variable.