gitlab-ci: tests medkits generation against HBK when default is HBS
We build only primary branches and we need build packages to test medkit generation. In most cases relying on default PUBLISH_BRANCH is exactly what we want but in case of HBK branch the target is correctly set to HBS. That is too far off branch and it makes more sense to just tests against HBK. This overrides tests to be executed against HBK if publish branch is HBS.