math/rand: make calls to Seed no-op

Makes calls to the global Seed a no-op. The GODEBUG=randseednop=0
setting can be used to revert this behavior.

Fixes #67273

Change-Id: I79c1b2b23f3bc472fbd6190cb916a9d7583250f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/606055


Auto-Submit: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>