decouple authentication information from HOMEBREW_ARTIFACT_DOMAIN

[UiP9AV6Y]

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

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[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @UiP9AV6Y!