Multilib processing fails when a build contains two platforms

[javihernandez]

As uncovered in this build, multilib processing is failing due to the following error:

asyncpg.exceptions.CardinalityViolationError: more than one row returned by a subquery used as an expression

The query in question is:

    subquery = (
        select(models.BuildTask.id)
        .where(
            sqlalchemy.and_(
                models.BuildTask.build_id == build_task.build_id,
                models.BuildTask.index == build_task.index,
                models.BuildTask.arch == "i686",
            )
        )
        .scalar_subquery()
    )

Since the build contains 2 platforms (and multilib applies to both), the subquery is returning 2 ids, one for Alma 8 and another one for Alma 9, while we expect to find exactly one.

I imagine that fixing the problem should be as simple as adding the condition models.BuildTask.platform_id == build_task.platform_id to the subquery, to ensure that we limit the results to the i686 packages of the same platform we are processing at that time.