.NET 9: Enable and better define "portable Linux" behavior

Source build should support both portable and distro-specific Linux build flavors. We'll also need to decide which is the default. There are reasons for both choices.

Today, we have the following dichotomy:

- Microsoft proprietary build produces the portable `linux` flavor.
- Source build produces distro-specific flavor, like `rhel`.

We need to support both flavors within source-build going forward. This issue goes into some detail on this topic: https://github.com/dotnet/runtime/pull/66132#issuecomment-1172679247

I'm also not aware of a good spec on portable linux. We should probably have one. It affects topics like `dotnet --info`, `dotnet build --use-current-runtime`, and (obviously) `dotnet restore`. More context: https://github.com/dotnet/sdk/issues/23539#issuecomment-1046587963.