PSCMake should use 'architecture' and 'toolset' values to prepare an environment for external strategy presets

[MarkSchofield]

As per the configure preset documentation, if 'architecture' and/or 'toolset' are set with an 'external' strategy, then PSCMake could prepare the environment for CMake - by launching 'vcvars' for the architecture/toolset values and launching cmake with that environment set.