libavif: disable gdk-pixbuf thumbnailer when cross-compiling

[lopsided98]

Description of changes

gdk-pixbuf doesn't support thumbnailers when cross-compiling, causing libavif to fail to cross-compile because it tries to wrap ${gdk-pixbuf}/bin/gdk-pixbuf-thumbnailer, which doesn't exist.

This doesn't cause any rebuilds when not cross-compiling.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
  • armv6l-linux (cross)
• 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.

cc @amjoseph-nixpkgs @mkg20001 @ncfavier

[lopsided98]

For reference, here is the discussion of why the thumbnailer is disabled when cross-compiling: https://gitlab.gnome.org/GNOME/gdk-pixbuf/-/issues/64

[ghost]

It would be super cute if, when cross-compiling, we dropped in a shell script where the thumbnailer ought to be, which echoed that link and then did an exit 1.

But maybe that's too cute.