.NET 8: Enable full linux-portable CI build

[mmitche]

Describe the Problem

Unified build seeks to expand the number of platforms that .NET 8 can source-build on, and replace the existing Microsoft official build with source-build. Part of this is enabling a build for the Linux portable assets we produce.

Describe the Solution

Enable a full linux-portable build of source-build in CI.

[MichaelSimons]

[Triage] @mmitche - Is this already covered by #2932? If not how is it different?

[mmitche]

Covered in that other issue