breeze-icons: generate icon cache

[ttuegel]

Motivation

Fixes #21345 by generating the icon cache when the theme is installed.

Testing

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[FRidh]

What if one uses other icon sets? (I don't)

[ttuegel]

What if one uses other icon sets? (I don't)

The default hicolor icon set is handled by the system environment builder. Other icon sets need to be handled in the same way by their maintainers. I imagine the Gnome icon themes are already treated this way.