eng/SourceBuild.props: when unset, let the repo determine RuntimeOS.

[tmds]

When source-build builds runtime, it will pass RuntimeOS and BaseOS.

When the repo is built directly using './build.sh --sb', let the repo determine those values rather than calculating them in SourceBuild.props.

Fixes #82160 (comment).

@ViktorHofer @oleksandr-didyk ptal.

[tmds]

For some extra context, ./build.sh --sb would previously do a portable build.

Since .NET 7, source-build no longer does portable builds and some changes were made to eng/SourceBuild.props to reflect that.

With those changes, the rid for RuntimeOS changed from linux to the target rid, which broke the ./build.sh --sb command as reported in #82160 (comment).

With this PR we defer to the runtime's behavior when DotNetBuildFromSource=true, which is to assume things get built using the default SDK from Microsoft, and therefore initialize RuntimeOS to the portable rid.

[tmds]

When source-build builds this, it sets both RuntimeOS and BaseOS: https://github.com/dotnet/dotnet/blob/3b38fd69120ec5cac0433923225a8d6f1d40c0bf/repo-projects/runtime.proj#L23-L24.

The changes here are for when eng/SourceBuild.props is used directly: instead of doing something specific, they defer to the repo to do something sensible.

@ViktorHofer can you merge this?

[ViktorHofer]

Thanks @tmds and sorry for the delay.