ci: Only run pages CI job on scheduled job runs

Since commit b9b7816e5a, the `pages` job
will still try to be run on `main` after an MR is merged, but will fail
because it depends on `coverage` and `style-check-advisory`, which are
no longer run on `main` after a merge.

See https://gitlab.gnome.org/GNOME/glib/-/pipelines/560680 for an
example failure.

Instead, make the `pages` job only run at the end of a scheduled CI run.
Its dependent jobs will have run then. This means that the ‘canonical’
code coverage report at
https://gnome.pages.gitlab.gnome.org/glib/coverage/ will be updated once
a week, rather than after every merge into `main`.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

.gitlab-ci.yml

@@ -659,6 +659,7 @@ scan-build:
       - "cov-int/build-log.txt"
 
 pages:
+  extends: .only-schedules
   stage: deploy
   only:
     - main