Motivation for this change

Krita 4.2.0 has been released.

Things done

• Updated version number and checksum
• Added new dependency quazip

Things to do

• we may need to update qt5, are these changes already in nixpkgs?
• update gmic_krita_qt, I should probably make a separate PR for that. (haven't used this so far, ?the current version seems to work with Krita 4.2.0 though?)
• DONE: I'm not really familiar with CMake magic, I've set QUAZIP_INCLUDE_DIR and QUAZIP_LIBRARIES as environment variables but they are still not found during build... the relevant CMake files can be found here and here.
• DONE: It now fails bceause there are ?missing functions? in quazip I'm an idiot - QUAZIP_LIBRARIES does not expect a directory.

• 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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.