Store channel instance derivations in a separate transaction

This means that these derivations are stored, even if a later part of the
process fails. Having the channel instance derivations stored might help work
out why the failure occurred, or better display information about it.

sqitch/revert/remove_guix_revisions_store_path.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:remove_guix_revisions_store_path from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;