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>
1 file changed