nixos/smokeping: replace smokeping.fcgi with a symlink

This fixes a regression from bb649d96b0.
There were permission problems, when the preStart script tried to copy
the smokeping.fcgi file over the old file.

nixos/modules/services/networking/smokeping.nix

@@ -299,7 +299,8 @@ in
         mkdir -m 0755 -p ${smokepingHome}/cache ${smokepingHome}/data
         rm -f ${smokepingHome}/cropper
         ln -s ${cfg.package}/htdocs/cropper ${smokepingHome}/cropper
-        cp ${cgiHome} ${smokepingHome}/smokeping.fcgi
+        rm -f ${smokepingHome}/smokeping.fcgi
+        ln -s ${cgiHome} ${smokepingHome}/smokeping.fcgi
         ${cfg.package}/bin/smokeping --check --config=${configPath}
         ${cfg.package}/bin/smokeping --static --config=${configPath}
       '';