Fix off-by-one in block packing lcli (#2878)

## Issue Addressed

The current `lcli` block packing code has an off-by-one where it would include an extra slot (the oldest slot) of attestations as "available" (this means there would be 33 slots of "available" attestations instead of 32).
There is typically only single-digit attestations remaining from that slot and as such does not cause a significant change to the results although every efficiency will have been very slightly under-reported.

## Proposed Changes

Prune the `available_attestation_set` before writing out the data instead of after.

## Additional Info

This `lcli` code will soon be deprecated by a Lighthouse API (#2879)  which will run significantly faster and will be used to hook into our upcoming monitoring platform #2873.

Author: macladson

lcli/src/etl/block_efficiency.rs

@@ -274,6 +274,9 @@ pub async fn run<T: EthSpec>(matches: &ArgMatches<'_>) -> Result<(), String> {
             // Add them to the set.
             included_attestations_set.extend(attestations_in_block.clone());
 
+            // Remove expired available attestations.
+            available_attestations_set.retain(|x| x.slot >= (slot.as_u64().saturating_sub(32)));
+
             // Don't write data from the initialization epoch.
             if epoch != initialization_epoch {
                 let included = attestations_in_block.len();
@@ -309,9 +312,6 @@ pub async fn run<T: EthSpec>(matches: &ArgMatches<'_>) -> Result<(), String> {
                     }
                 }
             }
-
-            // Remove expired available attestations.
-            available_attestations_set.retain(|x| x.slot >= (slot.as_u64().saturating_sub(32)));
         }
 
         let mut offline = "None".to_string();