borg.sh: only try ssh keys, not password authentication

borg.sh

@@ -12,7 +12,7 @@ if [ "$1" = "--rw" ] ; then
         echo "=== Need SSH key passphrase.  Check Bitwarden for:"
         echo "===   borg $HOSTNAME / read-write SSH key"
     fi
-    export BORG_RSH="ssh -F $SSH/config -o BatchMode=no -i $SSH/id_ecdsa"
+    export BORG_RSH="ssh -F $SSH/config -o BatchMode=no -o PreferredAuthentication=publickey -i $SSH/id_ecdsa"
     shift
 else
     export BORG_RSH="ssh -F $SSH/config -i $SSH/id_ecdsa_appendonly"