fix cost subqueryscan wrong parallel cost

The cost_subqueryscan function does not judge whether it is parallel.
regress
-- Incremental sort vs. set operations with varno 0
set enable_hashagg to off;
explain (costs off) select * from t union select * from t order by 1,3;
QUERY PLAN
----------------------------------------------------------
Incremental Sort
Sort Key: t.a, t.c
Presorted Key: t.a
-> Unique
-> Sort
Sort Key: t.a, t.b, t.c
-> Append
-> Gather
Workers Planned: 2
-> Parallel Seq Scan on t
-> Gather
Workers Planned: 2
-> Parallel Seq Scan on t t_1
to
Incremental Sort
Sort Key: t.a, t.c
Presorted Key: t.a
-> Unique
-> Sort
Sort Key: t.a, t.b, t.c
-> Gather
Workers Planned: 2
-> Parallel Append
-> Parallel Seq Scan on t
-> Parallel Seq Scan on t t_1
Obviously the latter is less expensive

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