ci-buildpackage: Ignore upstream gbp.conf

To avoid conflicts with any upstream gbp.conf that packages may ship, the GitLab pipeline currently sets GBP_CONF_FILES=/dev/null.

However, having it in the pipeline leads to confusing errors when testing things locally, as it easy to forget to set the right environment in the Docker container.

The apertis-pkg-pull-updates script already does something similar, only sourcing /etc/git-buildpackage/gbp.conf. We may later look into aligning them.

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