886481beb1
While the toolbox image itself has all necessary dependencies to build and run the shell, we still need them in the destdir when building a system extension. Achieve this by running .gitlab-ci/install-common-dependencies.sh if it exists, which is the script that is used both for gnome-os system extensions and mutter's CI image. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3498>