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/deploy/build_servers_lookup_builds.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:build_servers_lookup_builds to pg
+
+BEGIN;
+
+ALTER TABLE build_servers ADD COLUMN lookup_builds boolean NOT NULL DEFAULT TRUE;
+
+COMMIT;