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

[JanKrivanek]

Context

This was discovered here dotnet/msbuild#11057 (comment) 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

[marcpopMSFT]

@dotnet/kitten dsplaisted points out that this could be fixed centrally in msbuild. baronfel doesn't want us to wait for that but it might be worth exploring on the msbuild side.