refactor: convert `Kustomization` resource into unstructured map only once during variable substitution

Signed-off-by: Ramasai Venkatsitarambhaskar Tadepalli <ramasai.tadepalli@mongodb.com>

internal/build/build.go

@@ -520,13 +520,16 @@ func (b *Builder) do(ctx context.Context, kustomization kustomizev1.Kustomizatio
 		return nil, fmt.Errorf("kustomize build failed: %w", err)
 	}
 
-	for _, res := range m.Resources() {
-		// run variable substitutions
-		if kustomization.Spec.PostBuild != nil {
+	if kustomization.Spec.PostBuild == nil {
+		return m, nil
+	}
+
 	data, err := runtime.DefaultUnstructuredConverter.ToUnstructured(&kustomization)
 	if err != nil {
 		return nil, err
 	}
+	for _, res := range m.Resources() {
+		// run variable substitutions
 		outRes, err := kustomize.SubstituteVariables(ctx,
 			b.client,
 			unstructured.Unstructured{Object: data},
@@ -545,7 +548,6 @@ func (b *Builder) do(ctx context.Context, kustomization kustomizev1.Kustomizatio
 			}
 		}
 	}
-	}
 
 	return m, nil
 }