root Makefile: fixed minor bug in save_old_srv_binaries rule

Makefile

@@ -125,7 +125,7 @@ copy_config_srv:
 # Save the previous nexus-server binary (in case a revert were needed)
 save_old_srv_binaries:
 	@echo "Saving old server binaries"
-	@ln $(SERVER_BASEDIR)/bin/nexus-server $(SERVER_BASEDIR)/bin/nexus-server.old
+	@cp -p $(SERVER_BASEDIR)/bin/nexus-server $(SERVER_BASEDIR)/bin/nexus-server.old
 
 copy_binaries_srv:
 	@echo "Copying server binaries"