Fix macos self hosted labels

[wolfv]

cc @aktech

[wolfv]

@aktech there is also this code that always adds "self-hosted" as a label. Should we keep it?

    self_hosted_default = list(
        runs_on[data["build_platform"]]["self_hosted_labels"]
    )
    self_hosted_default += ["self-hosted"]

[isuruf]

This is correct AFAIK. cirun might be overriding defaults.

See for eg: https://github.com/guyrt/arrow/blob/master/dev/tasks/python-wheels/github.osx.arm64.yml#L44

[aktech]

@aktech there is also this code that always adds "self-hosted" as a label. Should we keep it?

Nope, that isn't required.