Windows does not allow to delete opened file.

Fixes #24704

Change-Id: Idfca2d00a2c46bdd9bd2a721478bfd003c474ece
Reviewed-on: https://go-review.googlesource.com/113935
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>