Don't try a transaction for the migrator on MySQL

As per https://dev.mysql.com/doc/refman/8.0/en/implicit-commit.html
CREATE TABLE statements automatically commit always. The only reason
this worked in the past was that PHPs PDO connection didn't check the
actual state on commit, but only checked their internal state.
But in PHP8 this was fixed:
https://github.com/php/php-src/blob/PHP-8.0/UPGRADING#L446-L450


So now commit() fails because the internal PDO connection implicitly
commited already.

Signed-off-by: Joas Schilling <coding@schilljs.com>