wrapGAppsHook4: fix gtk4 for the wrong system

[Artturin]

gtk3(gtk4) is in depsTargetTargetPropagated while without splicing this gtk3(gtk4) will be for hostTarget
and when wrapGAppsHook4 is in nativeBuildInputs
packages in depsTargetTargetPropagated will be treated like buildInputs

fixes pkgsCross.aarch64-multiplatform.gnome.zenity

Closes #238440

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[kirillrdy]

it builds !

pr-242720/nixpkgs]$ nix-build -A pkgsCross.aarch64-multiplatform.gnome.zenity
/nix/store/lhg42m18ayql73lrwqfrwkjqqdcnckmh-zenity-aarch64-unknown-linux-gnu-3.99.0

[github-actions]

Successfully created backport PR for release-23.05:

• [Backport release-23.05] wrapGAppsHook4: fix gtk4 for the wrong system #242743

[avnik]

@Artturin Could you explain, how __splicedPackages works, please?

[Artturin]

@avnik #204303