os: don't include DirFS argument in DirFS errors

Otherwise we wind up mixing GOOS paths with slash separated paths.

Change-Id: I63dd733cbdb0668effbc030cfd58945008732d9e
Reviewed-on: https://go-review.googlesource.com/c/go/+/446115


Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>