GenerateFunctionsExtensionsMetadata breaks when assembly references a .net core dll

Create an Azure 3.1 function.
Either

- reference Microsoft.Extensions.Configuration.UserSecrets package
- Use a [assembly: FunctionsStartup(..)]
or
- Include a UserSecretsId in the PropertyGroup for the Project configuration

When building because the Microsoft.Extensions.Configuration.UserSecrets dll is a framework dll, it is not copied to the output directory. When the task is run, as it is a NetFramework 4.6 task it cannot find the referenced dll, resulting in the startup extension is not written to extensions.json.