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.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -205,6 +205,14 @@ publish-documentation-builder-docker-image:
   variables:
     image: documentation-builder
 
+publish-testcases-builder-docker-image:
+  extends: .publish-docker-image
+  stage: publish images
+  needs:
+     - build-testcases-builder-docker-image
+  variables:
+    image: testcases-builder
+
 publish-image-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images