Unnest scan does not choose union scan when unnest predicate has OR clause

See CBSE-5634.

When a query has unnest, and unnest predicates has an OR clause with subterms corresponds to different indexes, in spock the planner uses an union scan, but in vulcan a primary scan is used.

Here is a simplified repro:

CREATE INDEX iax1 ON default(DISTINCT ARRAY v.x FOR v IN a1 END);

CREATE INDEX iax2 ON default(DISTINCT ARRAY v.y FOR v IN a1 END);

SELECT 1 AS a FROM default AS d UNNEST d.a1 AS v WHERE v.x = 10 OR v.y = 20;