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/verify/build_servers_lookup_builds.sql

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