Add a source-build leg to the MSFT VMR build that produces the source-built artifacts.

[mmitche]

It will be important that the MSFT and source build builds have matching official build IDs when generating the source-build artifacts. Therefore, it makes sense to move the source-build build that produces the shipping artifacts into MSFTs VMR build.

[MichaelSimons]

This is superseded by #4922.