First replace the "when: on_failure" rule by a "when: always". We indeed
always want to get log artifact so that we can study a build if
necessary (neither only on failure nor on success; really on all cases,
since even an apparently successful build may have issues we might want
to diagnose).

Also expire all artifacts at 1 week (it seems the default on GNOME's
Gitlab is 4 weeks; we don't need to keep these so long. Even a few days
might be enough actually).

As for the artifacts contents, keep the build dirs rather than the
install dirs. Build dirs allow to check configuration logs and other
kind of logs which are the most useful when diagnosing a failed build.
Now install dirs are also interesting. Maybe we should provide them
again at some point. We'll see. For now I comment them out.
Still keep the install dir for dependencies though, since it seems this
is how data are passed from one job to another.
Note that ideally we would like to provide different artifacts depending
on failure or success but apparently this is currently not possible.
See: https://gitlab.com/gitlab-org/gitlab/issues/18744

Also not sure why for GIMP, the CI was only keeping the build app/tests/
directory. We should really keep the whole dir.