ci: Install dependencies when using custom destdir

ci: Install dependencies when using custom destdir

Commit fd4320156c9 changed the `update-mutter` command inside the
toolbox to also install into the sysext extension dir, so people
who don't build mutter themselves can still use the sysext tooling.

Except they can't, because additional dependencies from mutter's
install-common-dependencies.sh script are missing as the script
is currently only invoked by the meson-build script.

Fix that by also running the script when running install-meson-project
with a destdir, as is done from the update-mutter command.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3805>