bug#41714: Some Flatpak applications fail to start when attempting to run xdg-dbus-proxy

[Jonathan Frederickson]

I have a few applications that I've installed through Flatpak on Guix 
System which used to work, but recently I've been unable to run them. 
Trying to run them results in:

jfred@lambdacrypt ~$ flatpak run im.riot.Riot
bwrap: execvp xdg-dbus-proxy: No such file or directory
jfred@lambdacrypt ~$ flatpak run org.signal.Signal
bwrap: execvp xdg-dbus-proxy: No such file or directory

This is my current Guix revision, and I've just run `guix system 
reconfigure` and updated my user packages:

jfred@lambdacrypt ~$ guix describe
Generation 25   Jun 03 2020 23:43:30    (current)
 guix 4c8700f
   repository URL: https://git.savannah.gnu.org/git/guix.git
   branch: master
   commit: 4c8700fd844d24f52c54966467765f14dca6510c

My current system config can be found here: 
https://raw.githubusercontent.com/jfrederickson/dotfiles/6d2724116a420b13adcf8c63afa7e5f197969669/guix/guix/system.scm