Allow TargetOS and TargetRid to be overridden in DotNetBuild.props

[jkoritzinsky]

This is needed to fix the linux-bionic builds in the VMR.

Contributes to dotnet/source-build#4955

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[ViktorHofer]

Doesn't this imply that due to L18, the passed in TargetOS value will never be used?

[jkoritzinsky]

Yes. The problem is that the runtime's build scripts process and change the passed in TargetOS when determining the RID. Because of our "nested inner build" logic, we effectively were stripping the "-bionic" part of "linux-bionic" from the inner build invocation and building the product in a weird blend of linux, linux-bionic, and android.

By letting this calculation happen again and overwriting the TargetOS based on the RID, we can restore the calculation of the TargetOS to the correct value by re-extracting it from the RID (which we calculate based on the originally-passed-in TargetOS).