Potential retirement of the macOS CI builder for GLib and GTK

The GNOME Foundation has been sponsoring a hosted macOS CI runner for a few years, but the admins are now planning to retire it for a couple of reasons: the hardware is getting long in the tooth (it’s still an x86_64 machine) it’s a shared bare metal environment, for licensing purposes (you can only virtualise macOS on an Apple machine running macOS), so CI pipelines compete for resources and tend to clobber each other unless the dependencies are shared or are completely isolated and never ins...