Update the build_servers_build_config values

Change target to '' if it matches system, as this matches the updated way of
storing derivations.

sqitch/verify/update_build_servers_build_config.sql

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