Support storing the id's representing builds in other services

Like Cuirass, or the Guix Data Service. This isn't so necessary with Cuirass,
as the derivation/output are unique identifiers, but for the Guix Build
Coordinator, one derivation/output can be built many times, with potentially
differing results.

sqitch/revert/build_add_build_server_build_id.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:build_add_build_server_build_id from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;