Change a constraint to add ON DELETE CASCADE

I've not used these in many places, to try and avoid hiding deleting data, but
in this case, this will allow more easily deleting the derivation source file
nars, by just deleting the derivation_source_files table entry.

sqitch/verify/change_derivation_source_file_nars_constraint.sql

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