avoid relocking head during builder health check (sigp#4323)

## Issue Addressed

sigp#4314 

## Proposed Changes

avoid relocking head during builder health check

## Additional Info

NA

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -5311,13 +5311,9 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     /// Since we are likely calling this during the slot we are going to propose in, don't take into
     /// account the current slot when accounting for skips.
     pub fn is_healthy(&self, parent_root: &Hash256) -> Result<ChainHealth, Error> {
+        let cached_head = self.canonical_head.cached_head();
         // Check if the merge has been finalized.
-        if let Some(finalized_hash) = self
-            .canonical_head
-            .cached_head()
-            .forkchoice_update_parameters()
-            .finalized_hash
-        {
+        if let Some(finalized_hash) = cached_head.forkchoice_update_parameters().finalized_hash {
             if ExecutionBlockHash::zero() == finalized_hash {
                 return Ok(ChainHealth::PreMerge);
             }
@@ -5344,17 +5340,13 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
         // Check slots at the head of the chain.
         let prev_slot = current_slot.saturating_sub(Slot::new(1));
-        let head_skips = prev_slot.saturating_sub(self.canonical_head.cached_head().head_slot());
+        let head_skips = prev_slot.saturating_sub(cached_head.head_slot());
         let head_skips_check = head_skips.as_usize() <= self.config.builder_fallback_skips;
 
         // Check if finalization is advancing.
         let current_epoch = current_slot.epoch(T::EthSpec::slots_per_epoch());
-        let epochs_since_finalization = current_epoch.saturating_sub(
-            self.canonical_head
-                .cached_head()
-                .finalized_checkpoint()
-                .epoch,
-        );
+        let epochs_since_finalization =
+            current_epoch.saturating_sub(cached_head.finalized_checkpoint().epoch);
         let finalization_check = epochs_since_finalization.as_usize()
             <= self.config.builder_fallback_epochs_since_finalization;