Fix is_builder_payment_withdrawable function (#4774)

We were checking if the builder had not been slashed or if the
withdrawable epoch was in the future compared to the current epoch.
We should check if the builder has not been slashed or if the current
epoch is equal to or has passed the withdrawable epoch.

Author: gitToki

specs/gloas/beacon-chain.md

@@ -754,7 +754,7 @@ def is_builder_payment_withdrawable(
     """
     builder = state.validators[withdrawal.builder_index]
     current_epoch = compute_epoch_at_slot(state.slot)
-    return not builder.slashed or builder.withdrawable_epoch >= current_epoch
+    return not builder.slashed or current_epoch >= builder.withdrawable_epoch
 ```
 
 ##### Modified `get_expected_withdrawals`