os: fix type check error in benchmark

Previously, 's' was only written to, never read,
which is disallowed by the spec. cmd/compile
has a bug where it doesn't notice this when a
closure is involved, but go/types does notice,
which was making "go vet" fail.

This CL moves the variable into the closure
and also makes sure to use it.

Change-Id: I2d83fb6b5c1c9018df03533e966cbdf455f83bf9
Reviewed-on: https://go-review.googlesource.com/108556
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/env_test.go

@@ -62,19 +62,24 @@ func TestExpand(t *testing.T) {
 	}
 }
 
+var global interface{}
+
 func BenchmarkExpand(b *testing.B) {
-	var s string
 	b.Run("noop", func(b *testing.B) {
+		var s string
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
 			s = Expand("tick tick tick tick", func(string) string { return "" })
 		}
+		global = s
 	})
 	b.Run("multiple", func(b *testing.B) {
+		var s string
 		b.ReportAllocs()
 		for i := 0; i < b.N; i++ {
 			s = Expand("$a $a $a $a", func(string) string { return "boom" })
 		}
+		global = s
 	})
 }