libavif: disable thumbnailer if !build.canExecute host

[ghost]

Description of changes

As suggested by @lopsided98 here:

• libavif: add pcre2 to fix cross #250610 (comment)

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.

[lopsided98]

Is !canExecute correct? Building the thumbnailer is controlled by meson.is_cross_build(). Also, I already submitted nearly the same PR, except I didn't use canExecute and avoid a mass-rebuild by keeping the blank line: #250813

[ghost]

Also, I already submitted nearly the same PR

Sorry I just noticed that. There is no backlink to it in #250610. Also I already had this commit in my local tree.

except I didn't use canExecute and avoid a mass-rebuild by keeping the blank line: #250813

Clever!