> What’s the output path for that?  (See "out" in
> /nix/store/jyj752q3jjxj0xjgm7wh2xwhkckndab7-coreutils-8.21.drv, for
> instance.)

Derive([("out","/nix/store/5sinjj4bra3lx812fiz6p46pq35qj16j-coreutils-8.21","","")] [...]

> Which Guix commit is it, and which platform?

91a7fde45654b2d78cd791542e6a18dbbedd7297; i686.

> Is the daemon doing chroot builds and build users?

Yes, AFAICT.  I use 'guix-daemon --build-users-group=nixbld'.

Any other ideas?