Github env variable support when rendering variables

[jnus]

You should be able to use the pre-existing GitHub environment variables, when specifying your variables in a json file. Both the GitHub and env context should be supported with the normal ${{ }} expression syntax.

E.g.

}
  "Name": "ApplicationSettings.Version",
  "Value": "1.0.0.${{ github.run_number }}",
  "Scope": {}
}