Since https://github.com/mroonga/mroonga/commit/35f593037c2d8f5a497349ff98f3a3ce31ee95de
install.sql has been split in install.sql and update.sql, adjust the
script to source both.

install.sql only contains INSTALL PLUGIN, it's safe to run it on every
update (that's what we always did).

This fixes a regression that mroonga user functions were no longer
available after d0589a41 (version up: MariaDB 10.3.35/10.4.25, groonga
12.0.4, mroonga 12.04, groonga-normalizer-mysql 1.1.8, 2022-06-10)