bug#77862: guix-daemon run as non-root sets up /etc/group incorrectly in build container

[keinflue]

When using the new ability of guix-daemon to run as non-root with the 
help of user namespaces, the testsuite of coreutils fails.

This is because the daemon incorrectly uses the host GID instead of the 
guest GID in the build container's /etc/group, which the testsuite uses 
to lookup the group's name via id -gn.