ci: Disable the macOS CI

It’s almost a complete waste of time at the moment. For several reasons,
jobs flakily fail on it more often than they succeed. It’s wasting
resources, slowing down development and making people quite frustrated.

 * https://gitlab.gnome.org/Infrastructure/GitLab/-/issues/627
 * https://gitlab.gnome.org/GNOME/glib/-/issues/2949
 * https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3462 and related
   test failures

Nobody has stepped up to deal with the test or CI runner flakiness, or
generally maintain this CI runner. If someone does care about preventing
regressions for GLib on macOS, and can put time into making the CI
reliable, then this commit can be reverted.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -505,7 +505,7 @@ freebsd-13-x86_64:
       - "_build/glib/glibconfig.h"
       - "_build/meson-logs"
 
-macos:
+.macos:
   extends: .only-origin
   stage: build
   tags: