dotnetup: Test that dotnetup telemetry gets sent via the .NET CLI, and vice versa

dotnetup and the dotnet CLI are supposed to use the same telemetry store and send each other's telemetry. However, we don't currently test this. It would be good to do so, though automated testing of this could be challenging.

See discussion here: https://github.com/dotnet/sdk/pull/55421#discussion_r3693799134

Idea for automated coverage: Instead of needing to build the .NET SDK, rely on the stage 0 version and test that it sends dotnetup telemetry. If it doesn't have the necessary test hooks (via environment variables), we'd need to add those so we could point it at a telemetry store and a mock telemetry endpoint.