Turn off innodb-snapshot-isolation.

This is causing some trouble on CI in our integration tests, e.g.
https://github.com/DOMjudge/domjudge/actions/runs/18819431955/job/53692609380:
```
2025-10-26T15:17:47.393646+00:00] request.ERROR: Uncaught PHP Exception Symfony\Component\HttpKernel\Exception\BadRequestHttpException: "["An exception occurred while executing a query: SQLSTATE[40001]: Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction"]" at ImportProblemService.php line 8 96 {"exception":"[object] (Symfony\\Component\\HttpKernel\\Exception\\BadRequestHttpException(code: 0): [\"An exception occurred while executing a query: SQLSTATE[40001 ]: Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction\"] at /opt/domjudge/domserver/webapp/src/Service/ImportProblemService. php:896)"} []
```

Related: https://github.com/DOMjudge/domjudge/issues/2848