Add System.Runtime.Loader to Build project

Author: gunndabad

src/GovUk.Frontend.AspNetCore.Build/GovUk.Frontend.AspNetCore.Build.csproj

@@ -21,6 +21,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
     <PackageReference Include="NuGetizer" Version="*" />
+    <PackageReference Include="System.Runtime.Loader" Version="4.3.0" />
   </ItemGroup>
 
 </Project>