msbuild: always use shared library from dotnet-sdk

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.

pkgs/development/compilers/roslyn/default.nix

···
     description = ".NET C# and Visual Basic compiler";
     homepage = "https://github.com/dotnet/roslyn";
     mainProgram = "csc";
-    platforms = platforms.linux;
     license = licenses.mit;
     maintainers = with maintainers; [ corngood ];
   };

pkgs/development/tools/build-managers/msbuild/default.nix

···
     </configuration>
   '';
 
+  inherit (stdenv.hostPlatform.extensions) sharedLibrary;
+
 in
 
 stdenv.mkDerivation rec {
···
     mv artifacts/msbuild artifacts/mono-msbuild
     chmod +x artifacts/mono-msbuild/MSBuild.dll
 
-    ln -s $(find ${dotnet-sdk} -name libhostfxr.so) artifacts/mono-msbuild/SdkResolvers/Microsoft.DotNet.MSBuildSdkResolver/
+    # The provided libhostfxr.dylib is for x86_64-darwin, so we remove it
+    rm artifacts/mono-msbuild/SdkResolvers/Microsoft.DotNet.MSBuildSdkResolver/libhostfxr.dylib
+
+    ln -s $(find ${dotnet-sdk} -name libhostfxr${sharedLibrary}) artifacts/mono-msbuild/SdkResolvers/Microsoft.DotNet.MSBuildSdkResolver/
 
     # overwrite the file
     echo "#!${stdenv.shell}" > eng/common/dotnet-install.sh
     echo "#!${stdenv.shell}" > mono/build/get_sdk_files.sh
+
+    # Prevent msbuild from downloading a new libhostfxr
+    echo "#!${stdenv.shell}" > mono/build/extract_and_copy_hostfxr.sh
 
     mkdir -p mono/dotnet-overlay/msbuild-bin
     cp ${dotnet-sdk}/sdk/*/{Microsoft.NETCoreSdk.BundledVersions.props,RuntimeIdentifierGraph.json} mono/dotnet-overlay/msbuild-bin
···
       --set-default MONO_GC_PARAMS "nursery-size=64m" \
       --add-flags "$out/lib/mono/msbuild/15.0/bin/MSBuild.dll"
 
-    ln -s $(find ${dotnet-sdk} -name libhostfxr.so) $out/lib/mono/msbuild/Current/bin/SdkResolvers/Microsoft.DotNet.MSBuildSdkResolver/
+    ln -s $(find ${dotnet-sdk} -name libhostfxr${sharedLibrary}) $out/lib/mono/msbuild/Current/bin/SdkResolvers/Microsoft.DotNet.MSBuildSdkResolver/
   '';
 
   doInstallCheck = true;