When migrating an array to a `multi` property, `array_unpack(.prop)` throws an error

[raddevon]

If a property is changed from an array to a multi property, a cast expression of array_unpack(.prop) should work, but instead, it throws an error:

SchemaError: possibly more than one element returned by the USING clause for the alteration of property 'names' of object type 'default::User', while a singleton is expected

• EdgeDB Version: 4.7+45e4acc
• EdgeDB CLI Version: 4.3.0-dev.1138+af2b269
• OS Version: macOS 14.0

Steps to Reproduce:

1. Start a new project
2. Migrate the before schema
3. Migrate the after schema
4. Enter the cast expression array_unpack(.names)

Before schema:

module default {
  type User {
    required names: array<str>;
  }
}

After schema:

module default {
  type User {
    required multi names: str;
  }
}