update image reference in gitlab-ci.yml for cross-repo pipeline testing

Benjamin Le Diguerherrequested to merge
bld/replace-image-by-variable into main

This MR is the result of a discussion with @loic.nicolle about potential cross-repo pipeline between sylva-projects/sylva-elements/container-images/trivy-libguestfs and diskimage-builder .

It simply replaces the CI image for a job with a variable defined in .gitlab-ci.yml.

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