Get proposer index via get_beacon_proposer_index (#5368)

Related to:

* #5364
* #5365

I merged these back-to-back without pulling in updates from master &
didn't realize that they conflicted. The first removes block as a
parameter which the second uses to get the proposer index. Thankfully,
the fix is simple; we can use `get_beacon_proposer_index` for this. It's
worth mentioning that `process_block_header` checks that these are the
same, so this change is safe.

Author: jtraglia

specs/gloas/beacon-chain.md

@@ -1483,7 +1483,7 @@ def process_execution_payload_bid(
                 amount=amount,
                 builder_index=builder_index,
             ),
-            proposer_index=block.proposer_index,
+            proposer_index=get_beacon_proposer_index(state),
         )
         state.builder_pending_payments[SLOTS_PER_EPOCH + bid.slot % SLOTS_PER_EPOCH] = (
             pending_payment