all system-libs CI leg

[tmds]

Currently the CI is always building with all bundled libs both for source-only as unified build.

It would be good to a have a CI for all system libs.

This doesn't have to be extensive. A single job on the vmr would already bring lots of coverage.

cc @MichaelSimons @omajid

[MichaelSimons]

[Triage] When updating, the https://github.com/dotnet/source-build/blob/main/Documentation/ci-platform-coverage-guidelines.md documentation should be updated to reflect this change.

We think an existing leg should be updated to use the system libs versus adding a new leg. Fedora and or Ubuntu feel like good candidates.

[MichaelSimons]

[Triage] This is a candidate we should consider for backporting to 9.0 depending on what changes were needed.