Check for dotnet.dll when enumerating SDK paths (#89333)

Check for dotnet.dll when enumerating SDK paths (#89333)

The host treats the existence of a versioned folder under sdk as a valid SDK path. We've seen this cause confusion as users can end up in this state without their knowledge (for example, uninstall leaves an empty folder - which would normally be harmless, except when we try to use it as a valid SDK).

This adds a check for dotnet.dll instead of just the existence of the directory.