ci: Run dist-job on a schedule as well as on tags

This will allow us to test that it actually still works, which is
important for being able to make releases, because once we push a tag,
there’s no going back. The release can’t happen if `dist-job` then fails
on that tag.

Sigh.

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

.gitlab-ci.yml

@@ -977,6 +977,7 @@ dist-job:
     - !reference [".with-git", "before_script"]
     - !reference [".build-gobject-introspection", "before_script"]
   only:
+    - schedules
     - tags
   script:
     - meson setup ${MESON_COMMON_OPTIONS}