ci: Temporarily only run the FreeBSD 13 CI on a schedule

Rather than on every commit, because the CI runner is currently broken:
https://gitlab.gnome.org/Infrastructure/GitLab/-/issues/594.

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

Helps: !3136

.gitlab-ci.yml

@@ -461,7 +461,10 @@ freebsd-12-x86_64:
       - "_build/meson-logs"
 
 freebsd-13-x86_64:
-  extends: .only-origin
+  # FIXME: Temporarily only run the FreeBSD 13 CI on a schedule, rather than on
+  # every commit to origin, because it’s broken:
+  # https://gitlab.gnome.org/Infrastructure/GitLab/-/issues/594
+  extends: .only-schedules
   stage: build
   tags:
     - freebsd-13