shell/caucase.sh: Assign value to intended variable.

Found by shellcheck.

shell/caucase.sh

@@ -121,7 +121,7 @@ unwrap () {
   printf "%s\\n" "$wrapped" | jq --raw-output .signature | \
     base64 -d > "$signature_file"
   payload="$(printf "%s\\n" "$wrapped" | jq --raw-output .payload)"
-  PUBKEY_FILE="$(mktemp --suffix=unwrap.pub)"
+  pubkey_file="$(mktemp --suffix=unwrap.pub)"
   if printf "%s\\n" "$payload" "$@" | openssl x509 -pubkey -noout > "$pubkey_file"; then
     printf "%s%s " "$payload" "$digest" \
     | openssl dgst \