Motivation for this change

Build has been broken since the libssh 0.7 -> 0.8 upgrade.
https://hydra.nixos.org/job/nixos/trunk-combined/nixpkgs.x2goclient.x86_64-linux

Note that this builds a non-release version, but has the added benefit of getting rid of the qt5 compatibility patch (and is probably preferrable to manually picking more patches to the 4 months old release).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.