lib/time: update tzdata to 2018g

Now that the tree has been frozen for some time, update the tzdata
database to version 2018g (released 2018-10-26) for Go 1.12.

Updates #22487

Change-Id: I9e82bcdaef28d308643c08c9fd3472e4c14a196e
Reviewed-on: https://go-review.googlesource.com/c/151299
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

lib/time/update.bash

@@ -8,8 +8,8 @@
 # Consult https://www.iana.org/time-zones for the latest versions.
 
 # Versions to use.
-CODE=2018e
-DATA=2018e
+CODE=2018g
+DATA=2018g
 
 set -e
 rm -rf work