wkhtmltopdf: use qt5's own mkDerivation

[ivanbrennan]

Motivation for this change

Currently, wkhtmltopdf fails at runtime as it cannot find the offscreen plugin:

wkhtmltopdf http://example.com example.pdf
qt.qpa.plugin: Could not find the Qt platform plugin "offscreen" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Aborted

Things done

Referencing #65399, used the Qt mkDerivation wrapper.

• 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 nix-review --run "nix-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.

Notify maintainers

cc @

[worldofpeace]

Works for me, thanks.