unityhub: fix editor with versions >= 2020.1.3f1 not opening

[tesq0]

Motivation for this change

Unity with versions >= 2020.1.3f1 installed through unityhub would crash when trying to open. Turns out libxml2 was missing.

Things done

• [ x] Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • [x ] 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.

[mkg20001]

Tested locally, editor works, merging