Generate and store system test derivations for all supported systems

Rather than just the native system. I'm not quite sure of the value here, as I
guess system tests should behave the same regardless of the way the software
is compiled, but this seems like it could be useful, and being explicit about
the system the derivation is for is good.

sqitch/verify/guix_revision_system_test_derivations_add_system.sql

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