Add REGISTRY_HOST and REGISTRY_GROUP vars in .gitlab-ci yml

What does this MR do and why?

In order to save cross-border bandwidth for JiHu, we'd like to switch gitlab-build-images in .gitlab/ci/*.yml from registry.gitlab.com to registry.gitlab.cn.

Steps:

• Add two vars, REGISTRY_HOST and REGISTRY_GROUP, in upstream repo they will be registry.gitlab.com and gitlab-org
• Replace registry.gitlab.com/gitlab-org/gitlab-build-images:[tag] with ${REGISTRY_HOST}/${REGISTRY_GROUP}/gitlab-build-images:[tag]
• In JiHu jh/.gitlab-ci.yml, define REGISTRY_HOST and REGISTRY_GROUP. This will be in JiHu MR.

This solution is discussed with Lin and Remy here.

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