explorerd: adjust log search for darkfid startup sync confirmation

This commit modifies the `wait_for_darkfid_startup` target in the Makefile to search for a later log message during darkfid startup. Results from testnet testing revealed that additional synchronization occurs before darkfid starts, necessitating a longer wait time is needed before starting explorerd.

bin/explorer/explorerd/Makefile

@@ -212,7 +212,7 @@ await-startup-no-sync-%:
 # Waits for Darkfid to start
 define wait_for_darkfid_startup
   log_dir=$(strip $(1)); \
-  while ! grep -q "Darkfid P2P handler started successfully!" "$$log_dir/darkfid.log" 2>/dev/null; do \
+  while ! grep -q "Blockchain synced!" "$$log_dir/darkfid.log" 2>/dev/null; do \
     sleep 1; \
   done;
 endef