Set Samba workgroup used by GTK applications to "localhost"

Otherwise, some GTK applications will resolve the "workgroup" hostname using Tor's DNS port, which can have a variety of adverse consequences. To avoid having to reason about these consequences, let's bypass the DNS resolution altogether.

This will break accessing folders and printers that are shared on the LAN using Samba, but we've never actively tried to support them anyway, and I suspect they're already broken because our firewall blocks netbios-ns.

I've tested this manually in a VM and it does solve #19030 (closed).

Closes #19030 (closed)