typo in updating DKIM, dont regenerate the DKIM private key each time setup is run

setup/dkim.sh

@@ -36,7 +36,7 @@ fi
 # mail.private and mail.txt in $STORAGE_ROOT/mail/dkim. The former
 # is the actual private key and the latter is the suggested DNS TXT
 # entry which we'll want to include in our DNS setup.
-if [ ! -z "$STORAGE_ROOT/mail/dkim/mail.private" ]; then
+if [ ! -f "$STORAGE_ROOT/mail/dkim/mail.private" ]; then
 	# Should we specify -h rsa-sha256?
 	opendkim-genkey -r -s mail -D $STORAGE_ROOT/mail/dkim
 fi