(chocolatey#2076) Pack always truncate stream

Mono does not truncate streams in System.IO.Packaging.Package.Open
when using FileMode.Create, while .Net does. mono/mono#21055

This always truncates the stream at the start.

src/chocolatey/infrastructure.app/nuget/NugetPack.cs

@@ -36,6 +36,11 @@ public static IPackage BuildPackage(PackageBuilder builder, IFileSystem fileSyst
             {
                 using (Stream stream = fileSystem.create_file(outputPath))
                 {
+                    // Truncate if needed, as Mono fails to truncate
+                    if (stream.Length > 0)
+                    {
+                        stream.SetLength(0);
+                    }
                     builder.Save(stream);
                 }
             }