blake2b: fix self-assignment in test

Fix self-assignment of useAVX variable. (vet warning)

Change-Id: I4194d7640f17cbf71dae27ce271d6f6d21354318
Reviewed-on: https://go-review.googlesource.com/43371
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

blake2b/blake2b_test.go

@@ -22,7 +22,7 @@ func fromHex(s string) []byte {
 
 func TestHashes(t *testing.T) {
 	defer func(sse4, avx, avx2 bool) {
-		useSSE4, useAVX, useAVX2 = sse4, useAVX, avx2
+		useSSE4, useAVX, useAVX2 = sse4, avx, avx2
 	}(useSSE4, useAVX, useAVX2)
 
 	if useAVX2 {