The build log of <https://ci.guix.gnu.org/build/2356870/details> has:

--8<---------------cut here---------------start------------->8---
guix system: [1m[0mbootloader successfully installed on '[1m/dev/vdb[0m'
+ sync
[  194.803139] udevd[313]: failed to execute '/gnu/store/pxw38aa6wwxyd2ws9pkzj4m68qrmjscd-eudev-3.2.9/lib/udev/${exec_prefix}/bin/udevadm' '${exec_prefix}/bin/udevadm trigger -s block -p ID_BTRFS_READY=0': No such file or directory
--8<---------------cut here---------------end--------------->8---

The broken udev rule is ‘64-btrfs.rules’ in eudev:

--8<---------------cut here---------------start------------->8---
# reconsider pending devices in case when multidevice volume awaits
ENV{ID_BTRFS_READY}=="1", RUN+="${exec_prefix}/bin/udevadm trigger -s block -p ID_BTRFS_READY=0"
--8<---------------cut here---------------end--------------->8---

… where “${exec_prefix}” is expanded from “@bindir@”, a classical
issue that we should report upstream.  :-)

In the meantime, the following patch should solve the problem.  I’m
running the test now, and if it passes and you’re fine with it, I’ll
commit it.

It’s pleasant we have tests to catch such issues!

Thanks,
Ludo’.