Add an index on the derivation_file_name field in the builds table

As this helps when finding builds relating to specific derivations.

sqitch/revert/add_index_on_builds_derivation_file_name.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:add_index_on_builds_derivation_file_name from pg
+
+BEGIN;
+
+DROP INDEX builds_derivation_file_name;
+
+COMMIT;