Define unit test to ensure that telemetry is not enabled in source-built SDK

Telemetry gets enabled in the SDK when the `OfficialBuilder` property is set to `Microsoft`. For the changes in https://github.com/dotnet/sdk/pull/48327, it actually caused the SB job that was included into the UB pipeline to have telemetry enabled. This is because it gets included in the same stage as the UB jobs. This happens because the `buildSourceOnly` parameter is `false` for the stage when importing the vmr-build.yml variables template: https://github.com/dotnet/sdk/blob/fc7f6e80ac1bd3de3269a98ff92f201116ac59f1/eng/pipelines/templates/variables/vmr-build.yml#L58-L65.

A unit test would have caught this issue.