os: use backslashes for DirFS on Windows

os: use backslashes for DirFS on Windows

Otherwise DirFS of a UNC path does not work.

Fixes #54694

Change-Id: I82c1c436f7c26b3935c2cc4fd238daf094fc4d86
Reviewed-on: https://go-review.googlesource.com/c/go/+/426094
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>