OutputPath (`-o`) option should ensure trialing slash

### Context

This was discovered here https://github.com/dotnet/msbuild/issues/11057#issuecomment-2567613668 by @KalleOlaviNiemitalo 

Implicitly defined OutputPath ensures trialing slash, while explicitly passed option doesn't hold the guarantee - this can lead to inconsistent behavior of downstream commands
