For source build on Windows we need to use the references that come from the downloaded cli

### Steps to reproduce

Revert PR: https://github.com/Microsoft/msbuild/pull/3340
Then go to dotnet/source-build repo. Checkout master branch and then update the src/msbuild git submodule to point to your branch that contains the reverted PR. To do this you need to push the branch to your fork and then in your source-build local repo from a command line go to src/msbuild and then run:

`git remote add myfork https://github.com//msbuild`
`git pull myfork `

navigate back to source-build root and then run:

`build.cmd /p:RootRepo=msbuild`

If you look at the restored Microsoft.NETCore.App package for the Microsoft.Build.csproj it will restore the version of the dotnet SDK that you have installed in your machine rather than using the one that comes from the downloaded CLI in source-build.

Note that in [source-build](https://github.com/dotnet/source-build/blob/master/build.ps1#L34) we're setting `DOTNET_MULTILEVEL_LOOKUP = 0` at the beginning of the execution to tell the cli to explicitly use the downloaded one.

Also, if this is the first time using the source-build repo, before doing anything else, you need to run:
`git submodule update --init --recursive`

cc: @rainersigwald