nixos/prometheus: fix blackbox exporter

nixos/modules/services/monitoring/prometheus/exporters/blackbox.nix

···
       throw
       "${logPrefix}: configuration file must not reside within /tmp - it won't be visible to the systemd service."
     else
-      true;
+      file;
   checkConfig = file:
     pkgs.runCommand "checked-blackbox-exporter.conf" {
       preferLocalBuild = true;