Use the (merge) commit sha when building CNG components

What does this MR do?

• The assets cache image is built against the merge commit (aka $CI_COMMIT_SHA): https://gitlab.com/gitlab-org/gitlab/-/blob/db839bf23e216b43e8b920265d7dc814a9365cc9/.gitlab/ci/frontend.gitlab-ci.yml#L58-63
• The CNG images are built against the branch: https://gitlab.com/gitlab-org/gitlab/-/blob/db839bf23e216b43e8b920265d7dc814a9365cc9/scripts/trigger-build#L181
• The problem is that the branch HEAD commit isn't the same as the merge commit, thus assets in the assets image have a high chance of having a different hash than the actual assets that would be generated from the branch HEAD.
• The correct assets are the ones generated from the merge commit, so we're changing the CNG trigger variables to pass GITLAB_VERSION=$CI_COMMIT_SHA instead of GITLAB_VERSION=$CI_COMMIT_REF_NAME so that the components are built from the same commit as the assets image.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Closes #202651 (closed).