lib/time, time/tzdata: update tz data to 2020b

Set ZFLAGS="-b fat" as the default was changed to '-b slim', see
http://mm.icann.org/pipermail/tz-announce/2020-October/000059.html
This will make sure that backwards-compatibibilty data is still emitted.

Updates #22487

Change-Id: I310a1b3a91e435673d1df41fbf8bb76abce1f94d
Reviewed-on: https://go-review.googlesource.com/c/go/+/261363


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>