Add an index on the derivation_file_name field in the builds table

As this helps when finding builds relating to specific derivations.

sqitch/verify/add_index_on_builds_derivation_file_name.sql

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