Fixup verifyWinePrefix for Proton... again

[CommandMC]

Proton copies over a ready-made default prefix when it detects that it needs to update, so running wineboot afterwards will always say that it wasn't updated. This changes verifyWinePrefix so it (1) returns early if a Proton prefix is already present & (2) returns true if Proton created a prefix

This is still somewhat naive since it relies on the pfx directory (not) being present, but it's still better than not working at all

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)