This changes the use of .so to use the more generic
stdenv.hostPlatform.extensions.sharedLibrary.
This also overrides mono/build/extract_and_copy_hostfxr.sh which uses
the internet and is x86 specific.
and do not add nuget.build.tasks.pack
```
/build/source/src/NuGet/Microsoft.Net.Compilers.Toolset/Microsoft.Net.Compilers.Toolset.Package.csproj : error NU1100: Unable to resolve nuget.build.tasks.pack (>= 5.10.0-preview.2.7169)' for '.NETCoreApp,Version=v3.1'.
/build/source/src/NuGet/Microsoft.Net.Compilers.Toolset/Microsoft.Net.Compilers.Toolset.Package.csproj : error NU1100: Unable to resolve 'nuget.build.tasks.pack (>= 5.10.0-preview.2.7169)' for '.NETFramework,Version=v4.7.2'.
```