meson: only update-icon-cache when the demos are built

Without building the demos, nothing gets installed into $prefix/share/icons/hicolor. Which makes running gtk4-update-icon-cache on the machine causes an error. This is easily reproducible on a Windows machine with MSVC where there is nothing pre-installed on hicolor icon theme and that makes building gtk without the demos fails with "No such file or directory".

Was reproduced on gtk4-rs is Windows CI https://github.com/gtk-rs/gtk4-rs/runs/4742617026?check_suite_focus=true and the issue is fixed if you build the demos, which is something we want to avoid as the build is very slow as is.