add support for credentials usage in combination with other mirroring
features. previously `HOMEBREW_DOCKER_REGISTRY_TOKEN` and
`HOMEBREW_DOCKER_REGISTRY_BASIC_AUTH_TOKEN` where only used in combination
with `HOMEBREW_ARTIFACT_DOMAIN` which affects *all* curl download strategies.
`HOMEBREW_BOTTLE_DOMAIN` is only used for bottle artifacts, whose mirror might
also require credentials.
this change removes the requirement of using `HOMEBREW_ARTIFACT_DOMAIN`.
related to #13226
