Add an extra index on blocked_builds

I think this might help with queries that don't use the build_server_id.

sqitch/verify/blocked_builds_blocked_builds_blocked_derivation_output_details_set_id_2.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:blocked_builds_blocked_builds_blocked_derivation_output_details_set_id_2 on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;