time: always use $GOROOT/lib/time/zoneinfo.zip with genzabbrs.go

genzabbrs.go uses whatever zoneinfo database available on the system.
This makes genzabbrs.go output change from system to system. Adjust
go:generate line to always use $GOROOT/lib/time/zoneinfo.zip, so it
does not matter who runs the command.

Also move go:generate line into zoneinfo.go, so it can be run
on Unix (see #16368 for details).

Fixes #15802.

Change-Id: I8ae4818aaf40795364e180d7bb4326ad7c07c370
Reviewed-on: https://go-review.googlesource.com/27832Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/zoneinfo.go

@@ -9,6 +9,8 @@ import (
 	"syscall"
 )
 
+//go:generate env ZONEINFO=$GOROOT/lib/time/zoneinfo.zip go run genzabbrs.go -output zoneinfo_abbrs_windows.go
+
 // A Location maps time instants to the zone in use at that time.
 // Typically, the Location represents the collection of time offsets
 // in use in a geographical area, such as CEST and CET for central Europe.

src/time/zoneinfo_windows.go

@@ -11,8 +11,6 @@ import (
 	"syscall"
 )
 
-//go:generate go run genzabbrs.go -output zoneinfo_abbrs_windows.go
-
 // TODO(rsc): Fall back to copy of zoneinfo files.
 
 // BUG(brainman,rsc): On Windows, the operating system does not provide complete