cmd/goimports: set correct permissions on Windows
cmd/goimports: set correct permissions on Windows

As of Go 1.14, WriteFile on Windows will set read-only on existing files
if you pass 0 for perms. Pass the pre-existing permissions.

Updates golang/go#38225.

Change-Id: I3174469efd4dc4c7eacc8522386a0712cfa39d11
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229297
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools