Start storing channel instance derivations

These are the ones that relate to Guix pull.

sqitch/deploy/channel_instance_derivations.sql

@@ -0,0 +1,13 @@
+-- Deploy guix-data-service:channel_instance_derivations to pg
+
+BEGIN;
+
+CREATE TABLE channel_instances (
+  guix_revision_id integer NOT NULL REFERENCES guix_revisions(id),
+  system varchar NOT NULL,
+  derivation_id integer NOT NULL REFERENCES derivations (id),
+  PRIMARY KEY (guix_revision_id, system),
+  UNIQUE (derivation_id)
+);
+
+COMMIT;