Allow anonymous access in private registries

[yahavi]

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Followup #11766

Currently when HOMEBREW_ARTIFACT_DOMAIN is configured and HOMEBREW_DOCKER_REGISTRY_TOKEN is not, a default QQ== access token is used to authenticate with private registries.
This PR change this behavior: If HOMEBREW_ARTIFACT_DOMAIN is configured, always use the token configured in HOMEBREW_DOCKER_REGISTRY_TOKEN, even if empty.

[MikeMcQuaid]

Great work again!