Two issues:
First, the tor project is asking distros not to build the tor browser
themselves. That includes Guix. That is because a build with Guix will make it
distinguishable from the tor-browser built by the tor project.
Second, the tor-browser built by the tor project assumes FHS (/bir, /lib, etc)
which guix doesn.t provide, so you can't run it directly.
Of course it's not great for anonymity since icecat is way more distinguishable
than the tor-browser built by guix would be.
You could use different profiles in icecat, one with tor settings, one without.
You can run icecat -P to select profiles.