bug#71962: Flatpak applications fail to launch with systemd-related error

[Juliana Sims]

How do you run flatpak in a system vm?

I've tried setting up flatpak inside the VM, which didn't work because 
I didn't persist the state; and exposing my user's home directory where 
all relevant flatpak files are stored, which gave an "unable to 
allocate session ID" error, which is distinct from the error I'm 
getting natively. I could try building an image with a persistent disk, 
but I figure if I'm running into issues running flatpak by sharing the 
relevant directories, making those directories over again inside the VM 
won't change things. I don't know how QEMU works well enough to noodle 
this one out on my own.

I also tested `guix shell --container' to ensure that cgroups work in 
other contexts, and they do. That combined with QEMU running fine leads 
me to eliminate user group membership and kernel configuration options 
(not that I've changed the kernel config from the default anyway) as 
potential causes of the error.

It's also worth noting that I'm experiencing the same issue on a 
different machine running Guix. They do share the base of their system 
configuration as well as the user configuration for my user account, 
though. I'll return to inspecting the configs for anything that might 
conflict.

Thanks for your ongoing help!