src/os: fix build tags for non-windows PathSeparator

src/os/path_unix.go

@@ -1,4 +1,5 @@
-// +build darwin linux,!baremetal
+//go:build !windows
+// +build !windows
 
 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style