Switch dotnet/scenario-tests onto the full code flow

[premun]

Context

We have agreed that we want to start dogfooding the flat dependency flow by using the dotnet/scenario-tests repository.
First, we need to implement #4158 though.

Goal

1. Disable synchronization of scenario-tests via the current pipeline method
2. Add a code-flow subscription from scenario-tests to the VMR
3. Add a build of the VMR so that we also have the backflow (can be empty for now)
4. Add a subscription from the VMR to the scenario-tests repository
5. Start building/publishing the scenario-tests packages out of the VMR
6. Disable publishing of the scenario-tests packages in scenario-tests's CI
7. Make repositories depending on scenario-tests depend on the VMR channel instead

[premun]

We can do steps 1 and 2 immediately after #4158 and and have one-way flow only

[mthalman]

The original issue body still mentions dotnet/sourcelink. That should be updated to scenario-tests.

[premun]

Forward flow now works: dotnet/dotnet#123
Backflow can only be enabled once we start producing packages from the VMR