Do not enforce the next version key if installing a snapshot.

Do not enforce the next version key if installing a snapshot.

Developers sometimes have dev machines with an older snapshot that
already has the correct signify key but sysupgrade(8) refuses to do an
upgrade because it thinks it's a version jump. That's just silly.

tb pointed out that signify(1) can just work out the correct key all
by itself.

problem reported, same diff & OK deraadt