Do not create the InnoDB temporary tablespace in innodb_read_only mode

The InnoDB temporary tablespace is only usable if innodb_read_only=OFF. It is useless to create the tablespace in read-only mode, because CREATE TEMPORARY TABLE is disallowed if innodb_read_only, and nothing can we written to the temporary tablespace if no temporary tables can be created.

(Note that in MySQL 5.7, optimizer-internal temporary tables might still use the InnoDB temporary tablespace even if innodb_read_only. In MariaDB the optimizer does not use InnoDB for its temporary tables, and MDEV-11487 removed the related InnoDB code.)