There is a wee typo in the systemd file guix-daemon.service installed on Ubuntu 20.04 by the guix-install.sh script.

The text of /root/.config/guix/current/lib/systemd/system/guix-daemon.service contains the text:

Environment='GUIX_LOCPATH=/var/guix/profiles/per-user/root/guix-profile/lib/locale' LC_ALL=en_US.utf8

The portion .../guix-profile/... should read .../.guix-profile/..., in other words the path as given omits the requisite single period (.) as the prefix of the directory .guix-profile.

This minor oversight accounts in part for the guile: warning: failed to install locale error and related warnings that have cropped up repeatedly when the build daemons are invoked

To test this fix:

I edited the guix-daemon.service specification in place at /etc/systemd/system by hand
Rebooted to restart the guix build daemons (I detest systemd and I'm lazy besides)
guix pull for my personal userland account
guix package -u to upgrade same
guix package -i libc-utf8-locales to upgrade my locales from 2.29 to 2.31
sudo -i guix pull to update the root's view of guix
sudo -i guix package -u for an upgrade of same
sudo -i guix package -i glibc-utf8-locales to have the latest version of the UTF-8 locales installed in ~root/.guix-profile/lib/locale

In subsequent user-level guix install X commands the warning no longer appears nor do the build daemons complain about failing to set the locale to en_US.utf8