> Can anyone reproduce the following?

Interesting. I replaced 'guix-daemon --build-users-group=nixbld' with
'nix-worker --daemon' and was able to build 'linux-libre-headers' as
well as 'coreutils' [1].  Any ideas?

[1] https://lists.gnu.org/archive/html/bug-guix/2013-06/msg00131.html