Pass $CI_COMMIT_TAG as GITLAB_TAG to the CNG triggered pipelines

What does this MR do?

This supports and must be merged before gitlab-org/build/CNG!192 (merged).

What are the relevant issue numbers?

Related to https://gitlab.com/charts/gitlab/issues/1110.

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer