readRandom doesn't matter on Linux because of startupRand, but it does
on Windows and macOS. Windows already uses the same API as crypto/rand.
Switch macOS away from the /dev/urandom read.
Updates #68278
Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64_14
Change-Id: Ie8f105e35658a6f10ff68798d14883e3b212eb3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/608436 Reviewed-by: Roland Shoemaker <roland@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>