time: look for zoneinfo.zip in GOROOT

The zoneinfo.zip file will be in the $GOROOT in self-hsoted builds
on iOS.

Updates #31722

Change-Id: I991fae92e3dc50581b099a2d8901aed36ecc7cef
Reviewed-on: https://go-review.googlesource.com/c/go/+/174310
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/zoneinfo_ios.go

@@ -7,10 +7,14 @@
 
 package time
 
-import "syscall"
+import (
+	"runtime"
+	"syscall"
+)
 
 var zoneSources = []string{
 	getZipParent() + "/zoneinfo.zip",
+	runtime.GOROOT() + "/lib/time/zoneinfo.zip",
 }
 
 func getZipParent() string {