Update BuildValidator to exclude refint folders.
Update BuildValidator to exclude refint folders.

Backport of https://github.com/dotnet/roslyn/pull/59462
weltkante/roslyn,dotnet/roslyn,shyamnamboodiripad/roslyn,bartdesmet/roslyn,jasonmalinowski/roslyn,jasonmalinowski/roslyn,CyrusNajmabadi/roslyn,jasonmalinowski/roslyn,CyrusNajmabadi/roslyn,dotnet/roslyn,bartdesmet/roslyn,weltkante/roslyn,shyamnamboodiripad/roslyn,dotnet/roslyn,mavasani/roslyn,mavasani/roslyn,weltkante/roslyn,bartdesmet/roslyn,mavasani/roslyn,shyamnamboodiripad/roslyn,CyrusNajmabadi/roslyn