ci: Manually fetch submodules for style-check CI jobs
This works around GitLab issue https://gitlab.com/gitlab-org/gitlab/-/issues/391756, which manifests as the error message:
Updating/initializing submodules...
Submodule 'subprojects/gvdb' (https://gitlab-ci-token:[MASKED]@gitlab.gnome.org/GNOME/gvdb.git) registered for path 'subprojects/gvdb'
Synchronizing submodule url for 'subprojects/gvdb'
fatal: not a git repository: subprojects/gvdb/../../.git/modules/subprojects/gvdb
on between 1/10 to 1/2 CI runs.
See the GitLab issue for a writeup.
Signed-off-by: Philip Withnall pwithnall@endlessos.org