MDEV-37905: UNCACHEABLE_RAND inconsistently set on unions

We need to ensure that all the unions are marked as UNCACHEABLE_RAND in
cases like:
  select * from t1 where 1 in (select 1 union select 2 union select RAND());

Author: DaveGosselin-MariaDB

sql/sql_lex.cc

@@ -11351,6 +11351,7 @@ SELECT_LEX_UNIT *LEX::parsed_select_expr_cont(SELECT_LEX_UNIT *unit,
   }
   last->link_neighbour(sel1);
   sel1->set_master_unit(unit);
+  unit->uncacheable |= sel1->uncacheable;
   sel1->set_linkage_and_distinct(unit_type, distinct);
   unit->pre_last_parse= last;
   return unit;