Instead of a convoluted fake rand, it is _basically_ just as fast, and
fixes errors that pop up due to bad entropy.

Fixes golang/go#70682

Change-Id: Ib0f605398d1092b516b03135f602c644be2a060f
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/633655
Reviewed-by: Tatiana Bradley <tatianabradley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>