Add a index to derivation_output_details_sets

This helps with queries like:

  SELECT *
  FROM derivation_output_details_sets
  WHERE ARRAY[5563227] <@ derivation_output_details_ids;

sqitch/verify/derivation_output_details_sets_derivation_output_details_ids_index.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:derivation_output_details_sets_derivation_output_details_ids_index on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;