[mono] Fix order of nuget sources, to avoid a bad package from
[mono] Fix order of nuget sources, to avoid a bad package from

.. dotnet-buildtools.

package: `microsoft.net.compilers 2.8.0`
The copy from `dotnet-buildtools` has missing files like
`Managed.Core.targets`.

The sources from `NuGet.config` are added after the ones in
`eng/Versions.props`, so we need add the api.nuget.org source in that
file.

mono/msbuild,mono/msbuild,mono/msbuild,mono/msbuild