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 <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Roland Shoemaker <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>