Backport of #83258. Should be low-risk backport because the service & package didn't exist in 19.09, nothing depends on them, and without this fix the module doesn't really work.

Motivation for this change

• The service has not been able to actually send emails because systemd sandboxing didn't allow it to call sendmail setgid wrapper. I probably broke it during the later iterations of sympa: init at 6.2.52 + NixOS module #65397 and haven't caught it until now. Shame on me.
• New version contains various bugfixes, including CVE-2020-9369. [not strictly needed to fix the problem]
• The --enable-fhs configure flag makes the resulting package layout saner, e.g. by not installing perl modules into $out/bin. Upstream plans to make this the default eventually. [not strictly needed to fix the problem]

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.