perf-test: update corpus signature

perf-test: update corpus signature

In the decade (!) since this corpus was last updated, the keyserver
network is essentially dead, and I have migrated gpg keys. Bump the
version number as a clean way of switching signatures. Also update the
instructions to suggest using "--locate-external-key" to download the
public key. By default this uses WKD, which is now supported for my
UID.