HostModel ships with the SDK which distributes the live version of System.Text.Json.

I tried to make HostModel reference this and hit a couple issues:

1. Referencing the live version caused official builds to rebuild parts of libraries during the installer test build - this broke because it made cross-gen rerun in a configuration where it wasn't able to execute.
2. Excluding the lib from the tests breaks when there's an observable change in the library (like assembly version, or API).

We should make it so that the installer subset can reference the live version and test with it, ideally without rebuilding libs.

As long as HostModel is on an old version we'll be chasing updates (due to CG / NuGet audit) and having to be careful not to break out of the version provided by VS.