CNG: Ensure UBI/FIPS builds consume "stable" images

Summary

Found in gitlab-org/build/CNG!1129 (comment 1229337501)

The .build-job-base templates do not consume the populate_stable_image_vars function. We should make the appropriate changes so that they do so in the future.

Steps to reproduce

Current behavior

Some build:* stages use images based on the Dockerfile, and do no populate from the xyz-stable images.

Expected behavior

xyz-stable are respected by all images for consistency and maintenance purposes.

Actionable

• Move xyz-stable jobs / phase-zero stage prior to prepare (this is where gitlab-uib-builder is)
• Integrate populate_stable_image_vars into .build-job-base templates of UBI and FIPS CI pipeline definitions.