lib/time: follow redirects in curl

Starting on or about the 2018c archives, www.iana.org is redirected to
data.iana.org.  Tell curl to follow the redirect.

Updates: #22487

Change-Id: I00acada1a3ba01ef701d6d4ffae6cc2cbb6a068f
Reviewed-on: https://go-review.googlesource.com/89375Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

lib/time/update.bash

@@ -16,8 +16,8 @@ rm -rf work
 mkdir work
 cd work
 mkdir zoneinfo
-curl -O http://www.iana.org/time-zones/repository/releases/tzcode$CODE.tar.gz
-curl -O http://www.iana.org/time-zones/repository/releases/tzdata$DATA.tar.gz
+curl -L -O http://www.iana.org/time-zones/repository/releases/tzcode$CODE.tar.gz
+curl -L -O http://www.iana.org/time-zones/repository/releases/tzdata$DATA.tar.gz
 tar xzf tzcode$CODE.tar.gz
 tar xzf tzdata$DATA.tar.gz