nixos/collectd: Fix package option

Commit ed979124cad7596de539188e86664b3784c363ca was missing some code.

nixos/modules/services/monitoring/collectd.nix

···
       wantedBy = [ "multi-user.target" ];
 
       serviceConfig = {
-        ExecStart = "${pkgs.collectd}/sbin/collectd -C ${conf} -P ${cfg.pidFile}";
+        ExecStart = "${cfg.package}/sbin/collectd -C ${conf} -P ${cfg.pidFile}";
         Type = "forking";
         PIDFile = cfg.pidFile;
         User = optional (cfg.user!="root") cfg.user;