MDEV-16735 mysql_upgrade failed

force alter_algorithm=DEFAULT in mysql_system_tables_fix.sql,
in case my.cnf sets it to something incompatible

scripts/mysql_system_tables_fix.sql

@@ -27,6 +27,7 @@
 set sql_mode='';
 set storage_engine=MyISAM;
 set enforce_storage_engine=NULL;
+set alter_algorithm=DEFAULT;
 
 ALTER TABLE user add File_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL;