Avoid duplicate build status entries

This effectively makes adding build status entries idempotent to some degree.

sqitch/deploy/build_status_add_unique_index.sql

@@ -0,0 +1,18 @@
+-- Deploy guix-data-service:build_status_add_unique_index to pg
+
+BEGIN;
+
+DELETE FROM build_status
+WHERE id NOT IN (
+  SELECT MIN(id) FROM build_status GROUP BY build_id, timestamp, status
+);
+
+ALTER TABLE build_status
+ADD CONSTRAINT build_status_build_id_timestamp_status_unique
+UNIQUE (build_id, timestamp, status);
+
+CREATE UNIQUE INDEX build_status_build_id_status_unique_idx
+  ON build_status (build_id, status)
+  WHERE timestamp IS NULL;
+
+COMMIT;