Deduplicate builds and add a unique index

Duplicate builds could creep in if the code to create them ran concurrently. I
didn't exclude them initially, as I was unsure if there should be such a
restriction, but at least for now, Cuirass builds map exactly to a single
derivation, so use the same restriction here.

sqitch/verify/sort_out_duplicate_builds.sql

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