Make `on_slot` return the block with the post header (#8188)

* Make `on_slot` return the block with the post header

Before this pr `on_slot` returned the pre block. However this is wrong,
because adding some post digest changes the hash of the header. Thus,
we need to make sure to return the correct block that uses the post
header.

* Update primitives/consensus/common/src/block_import.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>