VSTS cloud builds should set buildnumber variable

[AArnott]

The VSTS cloud builds have a Build.BuildNumber variable, as defined here.

Although NB.GV can set the build number, the Build.BuildNumber variable remains unchanged. Consider changing it explicitly within the build as well.

[AArnott]

I confirmed that without any change, the BUILD_BUILDNUMBER environment variable does change at least by the time the next task in the VSTS build definition executes. But perhaps the issue @kirankulkarni747 and I saw is because the BUILD_BUILDNUMBER environment variable is consumed by the same build task that invoked NB.GV. If that's true, then the fix is probably to have NB.GV set the environment variable directly in addition to emitting the special ##vso instruction.