Update CI hook name to local-pre-source-build-gitlab-ci

With 6075d4c221ad "Add a hook to run a custom child pipeline after the
source build" landed two years ago the proper name for the local
pre-build hook has changed, switch to it rather than relying on the
backward compatibility shim.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>