Change nars size to bigint

As some nars can be bigger than the maximum size of an int.

sqitch/deploy/change_nars_size_to_bigint.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:change_nars_size_to_bigint to pg
+
+BEGIN;
+
+ALTER TABLE nars ALTER COLUMN size TYPE bigint USING size::bigint;
+
+COMMIT;