engine-manager: mark the junction tests Windows-only the way the package does

The _windows suffix already limits the file to Windows builds. Every other OS-specific test in the package also states it with a //go:build line, so this one does too.

Signed-off-by: Terve <ntervalon@nvidia.com>

Author: Noah-Tervalon-Nvidia

services/nvpair-engine-manager/modelsdir_windows_test.go

@@ -1,6 +1,8 @@
 // SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+//go:build windows
+
 package main
 
 import (