Fix enum MixinEnvironment.Option parsing always using the default

[Player3324]

The previous implementation always throws since it passes the wrong class to Enum.valueOf (e.g. CompatibilityLevel$2 instead of CompatibilityLevel)

[LlamaLad7]

What real-world impact does this have?

[Player3324]

It only has an effect if someone uses the system property, but I'm mostly fixing this because it throws and I'm investigating a different IllegalStateException