test/errchk: use "#!/usr/bin/env perl" shebang line

fix test failure of go.tools sub-repo on NetBSD.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/9662044

test/errchk

-#!/usr/bin/perl
+#!/usr/bin/env perl
 # Copyright 2009 The Go Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.