Add missing COSTS OFF to EXPLAIN commands in rowsecurity.sql.

Commit e5e11c8c added a bunch of EXPLAIN statements without COSTS OFF
to the regression tests.  This is contrary to project policy since it
results in unnecessary platform dependencies in the output (it's just
luck that we didn't get buildfarm failures from it).  Per gripe from
Mike Wilson.

src/test/regress/sql/rowsecurity.sql

@@ -650,37 +650,37 @@ SELECT * FROM z1 WHERE f_leak(b);
 EXPLAIN (COSTS OFF) SELECT * FROM z1 WHERE f_leak(b);
 
 PREPARE plancache_test AS SELECT * FROM z1 WHERE f_leak(b);
-EXPLAIN EXECUTE plancache_test;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test;
 
 PREPARE plancache_test2 AS WITH q AS (SELECT * FROM z1 WHERE f_leak(b)) SELECT * FROM q,z2;
-EXPLAIN EXECUTE plancache_test2;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test2;
 
 PREPARE plancache_test3 AS WITH q AS (SELECT * FROM z2) SELECT * FROM q,z1 WHERE f_leak(z1.b);
-EXPLAIN EXECUTE plancache_test3;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test3;
 
 SET ROLE rls_regress_group1;
 SELECT * FROM z1 WHERE f_leak(b);
 EXPLAIN (COSTS OFF) SELECT * FROM z1 WHERE f_leak(b);
 
-EXPLAIN EXECUTE plancache_test;
-EXPLAIN EXECUTE plancache_test2;
-EXPLAIN EXECUTE plancache_test3;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test2;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test3;
 
 SET SESSION AUTHORIZATION rls_regress_user2;
 SELECT * FROM z1 WHERE f_leak(b);
 EXPLAIN (COSTS OFF) SELECT * FROM z1 WHERE f_leak(b);
 
-EXPLAIN EXECUTE plancache_test;
-EXPLAIN EXECUTE plancache_test2;
-EXPLAIN EXECUTE plancache_test3;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test2;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test3;
 
 SET ROLE rls_regress_group2;
 SELECT * FROM z1 WHERE f_leak(b);
 EXPLAIN (COSTS OFF) SELECT * FROM z1 WHERE f_leak(b);
 
-EXPLAIN EXECUTE plancache_test;
-EXPLAIN EXECUTE plancache_test2;
-EXPLAIN EXECUTE plancache_test3;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test2;
+EXPLAIN (COSTS OFF) EXECUTE plancache_test3;
 
 --
 -- Views should follow policy for view owner.