In #222 I added case normalisation to ReadAction() to ensure that Docker and docker are interpreted the same way.
I realised that this was being done at the wrong level and required multiple type conversions. By implementing `func (a ActionRunsUsing) UnmarshalYAML` we can lowercase the string as it's being unserialized
This has an added benefit that any time this type is hydrated the `runs.using` key will be lowercased, rather than relying on `ReadAction()`
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