/etc/nix/nix.conf is a static file doesn't need rebuild on every boot.
* gnu/services/nix.scm (nix-activation): Don't create /etc/nix/nix.conf.
(nix-service-etc): New function.
(nix-service-type): New service-extension "nix-service-etc" to etc-service-type.
Signed-off-by: 宋文武 <iyzsong@member.fsf.org>