Pipeline Does MGCB Search

After converting the tools over to use Protobuild, the Pipeline and MGCB executables began building into their own separate folders.  This means that by default Pipeline can't find the location of MBCB when running from the normal build locations.

This PR adds a simple search for the MGCB.exe in a few known locations.  This solves our immediate issue and can be expanded later for Mac, Linux, environment vars, regkeys, etc.