Fix NuGet restore error
Fix NuGet restore error

Restoring the mono/build/sdks_and_nugets/sdks/sdks.csproj would
fail if the nuget packages were not in the cache with errors:

Package System.Runtime.CompilerServices.Unsafe 5.0.0 is not
compatible with net40 (.NETFramework,Version=v4.0) / win-x86

Fix this by having sdks.csproj target .NET 4.7.2 instead of
netcoreapp2.0 which was not being resolved.

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