v2021pre: gitlab-ci: Actually publish the testcases-builder image

The job adding the latest tag for the testcases-builder image was missing. Things still worked because the existing tags was still available and it's not an image which has seen much activity, but now that we branched v2021pre and v2022dev0 the pipeline rendering the test cases was failing to find the images.

Backport of !130 (merged)