Pull in nix-daemon.socket

This led to the container test failing, which made no sense
whatsoever, until I realized nix-daemon.socket creates the socket
directory as a side effect, which systemd-nspawn then bind-mounts.

http://hydra.nixos.org/build/10397575

nixos/modules/services/misc/nix-daemon.nix

@@ -277,6 +277,8 @@ in
 
     systemd.packages = [ nix ];
 
+    systemd.sockets.nix-daemon.wantedBy = [ "sockets.target" ];
+
     systemd.services.nix-daemon =
       { path = [ nix pkgs.openssl pkgs.utillinux pkgs.openssh ]
           ++ optionals cfg.distributedBuilds [ pkgs.gzip ];