lib/time, time/tzdata: update tzdata to 2020f

Changelog

 'make rearguard_tarballs' no longer generates a bad rearguard.zi,
 fixing a 2020e bug.

No actual changes to timezones data. See

  http://mm.icann.org/pipermail/tz-announce/2020-December/000064.html

Updates #22487

Change-Id: I78f7adba1c3c1d3489b0da870601117b9b8cb0d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/280455


Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>