Draft: CI: Upgrade Meson to 0.64.0 for macOS and MSVC builds
GTK uses GLib from git main, which requires Meson 0.64.0 since glib!3077 (merged).
This hopefully fixes the CI failures seen on !5248 (merged). If you would prefer not to upgrade Meson, the other way to do this would be to pin the GLib version to 2.74.x (same as !5252 (merged)), but in this MR I'm assuming that GTK main is intentionally using GLib main.