Create a table for systems

And use it for the systems in the derivations and package derivations tables.

The primary motivation here is to allow quickly working out what systems the
database contains, and having a small table with just the right data seems a
good way to do that.

sqitch/verify/systems_table.sql

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