blake2b: fix self-assignment in test
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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/crypto,golang/crypto,ScriptRock/crypto,golang/crypto,ScriptRock/crypto