ci: Instruct gitlab to checkout submodules

ci: Instruct gitlab to checkout submodules

By default, gitlab skips checking out of submodules. So far this
didn't matter to us, as meson will initialize submodules as part
of setup.

It no longer does for the dist command however, and as we run it
from a separate job where the submodule checkout isn't preserved,
the job breaks with recent meson versions.

Fix this by instructing gitlab to checkout the submodule for all
jobs that need it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2371>