$CI_TEMPLATE_REGISTRY_HOST is hard-coded to registry.gitlab.com

This CI variable $CI_TEMPLATE_REGISTRY_HOST is hard-coded as registry.gitlab.com in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/project.rb?ref_type=heads#L2856

We're required to copy components to our self-hosted instances to be able to use them, which is quite cumbersome, but even though images are being built and hosted on our own registry, these are being pulled to registry.gitlab.com.