Fixes #15302

The subquery on the users table was incorrectly using the username from the workspaces table, not the users table.

This passed sqlc-vet because the column did exist in the query, it just was not the correct one.

Note: If we do not care about deleted users, we can drop querying the Users table at all and use the owner_username on the workspaces. But then we'd get deleted users with the same name 😢