Do not check CHANGES in pre-release pipelines

Do not check CHANGES in pre-release pipelines

Since pre-release testing is usually carried out for branches in which
CHANGES entries are intentionally malformed to prevent entry numbering
conflicts down the road, do not run the "changes" GitLab CI job in
pipelines that are triggered by a parent pipeline (which can currently
only be a pre-release testing pipeline) to prevent triggering job
failures that would be meaningless anyway.

(cherry picked from commit 8c2503947f7ddea3437f89e8974690858736cb86)