EXPLAIN EXECUTE doesn't support coordinator INSERT/SELECT with parametrized SELECT.

```sql
CREATE TABLE simple (id integer, name text);
SELECT create_distributed_table('simple', 'id');
PREPARE insert_via_coordinator AS INSERT INTO simple SELECT count(*), name FROM simple WHERE name IN ($1, $2) GROUP BY name;
EXPLAIN EXECUTE insert_via_coordinator('x', 'y');
```

produces the following error:

```
ERROR: could not create distributed plan
DETAIL: Possibly this is caused by the use of parameters in SQL functions, which is not supported in Citus.
HINT: Consider using PL/pgSQL functions instead.
```