Don't assume that BaseOS has a value

[jkoritzinsky]

Sometimes we can end up building a non-portable build with a portable rid and without specifying a BaseOS value. Handle that in the installer targets logic.

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[akoeplinger]

Thanks. Once this is built we should update just the Microsoft.DotNet.Build.Tasks.Installers package version in your runtime PR since the regular arcade bump will bring in the net10 change.

[jkoritzinsky]

@akoeplinger I don't think we'll be able to merge with just updating that package (as it will cause prebuilt warnings) but I can give it a shot