Ensure that nscd, sshd are created as system users

c0f70b4694bb2be1c25ef0e23e3f11f9d718d162 removed the fixed uid
assignment, but then it becomes necessary to set isSystemUser.

http://hydra.nixos.org/build/22182588

nixos/modules/services/networking/ssh/sshd.nix

···
   config = mkIf cfg.enable {
 
     users.extraUsers.sshd =
-      { description = "SSH privilege separation user";
-        home = "/var/empty";
+      { isSystemUser = true;
+        description = "SSH privilege separation user";
       };
 
     environment.etc = authKeysFiles ++ [

nixos/modules/services/system/nscd.nix

···
 
   config = mkIf cfg.enable {
 
-    users.extraUsers.nscd.description = "Name service cache daemon user";
+    users.extraUsers.nscd =
+      { isSystemUser = true;
+        description = "Name service cache daemon user";
+      };
 
     systemd.services.nscd =
       { description = "Name Service Cache Daemon";