Use pipeline variable to reference MAR Status API resource/scope

Currently we hardcode the resource scope for the MAR status API: https://github.com/dotnet/docker-tools/blob/7b8f03267f6687fd6ecf7bca1071f74536c1cfa6/src/Microsoft.DotNet.ImageBuilder/src/McrStatusClient.cs#L20

This is very inflexible and requires changes to our source code if we need to change where this points to. Instead, we should accept a scope via the CLI and pass it in via an environment variable so that we can change it as needed without rebuilding ImageBuilder. We don't intend to frequently access this API on dev machines, so this should be an acceptable solution.