[8.x] BelongsToMany->sync() will support touching for pivots when the result contains detached items

[christoph-kluge]

Relates to #38084.

While running the following code $news was not touched. This PR will add support when $touching is correctly defined on the pivot.

$news->authors()->saveMany([
    new User(['id' => 1]),
    new User(['id' => 2]),
    new User(['id' => 3])
])
$news->authors->sync([1,2]);