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

[mthalman]

Telemetry gets enabled in the SDK when the OfficialBuilder property is set to Microsoft. For the changes in dotnet/sdk#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.

[omajid]

We use the following to test this for .NET source-build on Linux: https://github.com/redhat-developer/dotnet-regular-tests/blob/main/telemetry-is-off-by-default/test.sh