time: use offset and isDST when caching zone from extend string

If the current time is computed from extend string
and the zone file contains multiple zones with the
same name, the lookup by name might find incorrect
zone.

This happens for example with the slim Europe/Dublin
time zone file in the embedded zip. This zone file
has last transition in 1996 and rest is covered by
extend string.
tzset returns IST as the zone name to use, but there
are two records with IST name. Lookup by name finds
the wrong one. We need to check offset and isDST too.

In case we can't find an existing zone, we allocate
a new zone so that we use correct offset and isDST.

I have renamed zone variable to zones as it shadowed
the zone type that we need to allocate the cached zone.

Fixes #45370

Change-Id: I79102e4873b6de20d8a65f8a3057519ff5fae608
Reviewed-on: https://go-review.googlesource.com/c/go/+/307190


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>