Follow up to #41342.

The reason why the problem of #41342 occurred is that unlike `create`,
`build` did not support the creation of multiple records, so it did not
address the problem.

It is weird that `post.commnets.where(foo: "bar").build([obj1, obj2])`
is allowed but `Comment.where(foo: "bar").build([obj1, obj2])` is not
allowed.

To avoid the confusion, it allows `build` multiple records even on non
association relation.