Pass GITLAB_REF_SLUG to CNG triggered pipelines

What does this MR do?

This will allow the CNG pipelines to use GITLAB_REF_SLUG instead of GITLAB_{CE,EE}_VERSION. GITLAB_REF_SLUG is set to master for QA branches since the gitlab:assets:compile job isn't run for these branches.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer