Add a CI job that does a 2 stage build of msbuild, the second stage with bootstrapped -mt mode

[JanProvaznik]

Summary

add validating msbuild can build itself with -mt mode

Background and Motivation

The purpose is an an end to end test for validating -mt mode migrations

Proposed Feature

.vsts-dotnet-ci.yml defines required PR checks
Add one that first builds msbuild with build.cmd, and then builds another copy with -mt mode, it should have the similar config to Windows core job

Ideally it would be extensible to be able to include another repository to build with -mt mode

Alternative Designs

No response