Add a lookup_builds field to the build_servers table

This is to allow for build servers where only the substitutes should be
queried, and it shouldn't be assumed that they're running Cuirass.

sqitch/revert/build_servers_lookup_builds.sql

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