A user application can now use os.Setenv("ZONEINFO", ..) becase the
value is no longer read on init of the time package.

Fixes #18619

Change-Id: Id8e303d67e6fb9c5d6ea9f969d8c94f6fff1bee3
Reviewed-on: https://go-review.googlesource.com/35639Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>