Got the same thing. The issue arises in commit 6edacf24911, so I was able to avoid the error by building from a commit before the syslog service was changed.
Alright, I guess the *real* issue is in which version of the `guix` command is being used. I had just installed this instance of Guix using the installer script, so the version used for the initial pull (`/usr/local/bin/guix`) was still `hash`ed. Maybe another logout would have solved my issue, but is was able to get around it by `hash`ing `~/.config/guix/current/bin/guix`.