Allow the passing in of msbuildEngine to msbuild.ps1

[michellemcdaniel]

In repos that need to mix vs and dotnet msbuild engines, and use this script directly, we need to allow for the msbuildEngine to be passed in.

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation