fix light client topic subscriptions (#6351)

Using `let contextFork = consensusFork` no longer seems to work to avoid
capturing the `var` loop variable; it ends up being `Electra` for all
handlers. Use `closureScope` as a more sustainable fix.

beacon_chain/light_client.nim

@@ -342,8 +342,9 @@ proc installMessageValidators*(
   for consensusFork in ConsensusFork:
     withLcDataFork(lcDataForkAtConsensusFork(consensusFork)):
       when lcDataFork > LightClientDataFork.None:
+        closureScope:
           let
-          contextFork = consensusFork  # Avoid capturing `Deneb` (Nim 1.6)
+            contextFork = consensusFork
             digest = forkDigests[].atConsensusFork(contextFork)
 
           # light_client_optimistic_update