Make a constraint deferrable

To allow for deferring it in a transaction when deleting derivations.

sqitch/deploy/make_some_constraints_deferrable.sql

@@ -0,0 +1,8 @@
+-- Deploy guix-data-service:make_some_constraints_deferrable to pg
+
+BEGIN;
+
+ALTER TABLE derivations_by_output_details_set
+    ALTER CONSTRAINT derivations_by_output_details_set_derivation_id_fkey DEFERRABLE INITIALLY IMMEDIATE;
+
+COMMIT;