[release-branch.go1.16] time: fix looking for zone offset when date is close to a zone transition

The old implement passed start - 1 or end in func lookup to adjust the offset.But if the time is close to the last zoneTrans, like the issue, testcase and comment, the "start" from lookup will be omega. It can't be adjusted correctly.

Fixes #49406

Change-Id: Ibaf82dc4db6d5dd3279796f003d2b19c38a26341
Reviewed-on: https://go-review.googlesource.com/c/go/+/360616


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Robert Findley <rfindley@google.com>
(cherry picked from commit 90462dfc)
Reviewed-on: https://go-review.googlesource.com/c/go/+/361954


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>