Thanks!  Sadly your patch does not fix the issue for me.

You can reproduce with

--8<---------------cut here---------------start------------->8---
./pre-inst-env guix environment --pure --ad-hoc blueman -- blueman-tray
--8<---------------cut here---------------end--------------->8---

I had a quick look at the package definition: it's a
glib-or-gtk-build-system but the package is in python, so you need to
set the PYTHONPATH manually I think.  Look at the other GNOME package
written in Python, my guess is that it's the same thing.

-- 
Pierre Neidhardt
https://ambrevar.xyz/