fix for WrapMysqlModifySubqueryTransformation when using join builder

[marcinlee]

Reiteration of: #6

Fixes error where .onIn()/.andOnIn() causes:
TypeError: parentQuery.isUpdate is not a function

This happens when we provide .onIn() with an objection query.

BlogPost.query(db)
  .alias('post')
  .innerJoin('post.comment', (join) => {
    join.onIn(
      'comment.author_id',
      Author.query(db).where('active', true)
    );
  });

Since WrapMysqlModifySubqueryTransformation does not make sense for join query, we simply skip it for JoinBuilder.

Note: This wasn't an issue in objection v1 and this error does not happen if we provide .onIn() with knex query instead.