The blake2b package only includes an asm implementation for amd64. Other
architectures use a generic Go implementation. Unfortunately appengine
is one of those architectures and we explicitly ignore appengine when
vendoring because it is an unsupported platform for Nomad.

Commit 925ec07a started ignoring appengine to help trim down the vendor
package. We now need to include appengine again - despite not building
for it - just to avoid accidently breaking other archs we do support.