time: correct path to time zone zip file on Unix

time: correct path to time zone zip file on Unix

Most Unix systems have their own time zone data,
so we almost never get far enough in the list to
discover that we cannot fall back to the zip file.
Adjust testing to exercise the final fallback.

Plan 9 and Windows were already correct
(and are the main users of the zip file).

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/19280043