refactor: clarify GOOS detection (#39620)

Introduce `consts.IsWindows`, now it's clearer to see how Windows build works

modules/consts/runtime.go

@@ -0,0 +1,8 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package consts
+
+import "runtime"
+
+const IsWindows = runtime.GOOS == "windows"