Move official build yaml from phases to jobs

[eerhardt]

We need to make the same change as #3908 to the official build yaml file:

https://github.com/dotnet/machinelearning/blob/master/build/vsts-ci.yml

"phases" in AzDO yaml files are the old schema. We should be using "jobs", which is the "new" schema.

See https://github.com/dotnet/arcade/blob/master/Documentation/AzureDevOps/PhaseToJobSchemaChange.md for more info.

[codemzs]

fixed by #3908

[eerhardt]

There are 2 yaml files in the repo - PR validation and the official build. #3908 only changed the former. This issue is to make the same change to the latter.