Not able to set permittedInsecurePackages

[Nokogiri]

I recently encountered the problem that I have to permit an unsecure package as it is needed as a dependency for a unfree package without source access.
The problem is it doesn't seem to matter where i put nixpkgs.config.permittedInsecurePackages = [ "dotnet-runtime_7" ];
in my config, nix still throws an error that dotnet-runtime_7 is unsecure.

So it seems the setting has no effect.

Is there any other way to permit unsecure packages?