Ensure the shared framework build uses the generated runtime.json

[ericstj]

Fixes #53550

[ViktorHofer]

By switching to the live generated runtime.json file, you are depending on the file already being generated which isn't the case until the Microsoft.NETCore.Platforms project is built. We should make sure that any code path that depends on it has a dependency to the Platforms project.

[ericstj]

Closing this as I don't have cycles to drive it at the moment. @tmds feel free to fix this as it best fits the work you're doing.

[tmds]

@ericstj the minimal thing we need for source-build is: have the non-portable rid show up in $DOTNET_ROOT/shared/Microsoft.NETCore.App/7.0.0/Microsoft.NETCore.App.deps.json.
Do you know where this file gets generated? Can we apply a patch so that picks up the runtime.json file from Microsoft.NETCore.Platforms.csproj?

[ericstj]

There are two places that generate a shared framework deps file. One is pretest.proj which is used by library testing.

runtime/src/libraries/pretest.proj

Line 97 in 1f1231c

RuntimeGraphFiles="$(RuntimeIdGraphDefinitionFile)"

The other is the shared framework SDK which uses the BundledRuntimeIdentifierGraphFile property https://github.com/dotnet/arcade/blob/9bbfa8dbc89c406eb8e2bc62600a007882ccd0ab/src/Microsoft.DotNet.SharedFramework.Sdk/targets/sharedfx.targets#L197
This is the one the is used inside the final shared-framework / runtime-packs cc @jkoritzinsky @NikolaMilosavljevic