cmd/go: remove renameio package and its last usage

The last primary usage of renameio was the WriteFile in
modfetch.WriteDiskCache. Because it's not guaranteed that the fsync in
WriteDiskCache will eliminate file corruption, and it slows down tests
on Macs significantly, inline that last usage, removing the fsync.

Also, remove the uses of renameio.Pattern. The ziphash file is no
longer written to a temporary location before being copied to its
final location, so that usage can just be cut. The remaining use is
for the zipfile . Remove the first because the files are no longer
written using the pattern anyway, so that the pattern variable has no
effect. Replace it with a local pattern variable that is also passed
to os.CreateTemp.

Change-Id: Icf3adabf2a26c37b82afa1d07f821a46b30d69ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/301889


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>