fix: fix prisma#17574 (#5516)
The custom schema provided in the URL will never exist in a newly
creeated shadow database. This change makes sure we create it.
[ORM-1097](https://linear.app/prisma-company/issue/ORM-1097/19677-sqlserver-shadow-db-creation-issues-with-custom-schema-name-in)
Fixes https://github.com/prisma/prisma/issues/17574
