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()`
Currently, providing `using: Docker` is an error as the switch statement in `step_context.go` is case sensitive.
Actions run successfully on GitHub with `using: Docker` leading me to believe that they're case insensitive. This commit updates `act` to match
Co-authored-by: Casey Lee <cplee@nektos.com>
