Use numbers for ids in the derivation model

As this is easier to handle when doing queries with PostgreSQL, as you can use
the type information to determine how to handle the values.

guix-data-service/model/derivation.scm

@@ -419,7 +419,9 @@ ORDER BY derivations.system DESC,
    #t "debug: insert-missing-derivations: inserting ~A derivations\n"
    (length derivations))
   (let ((derivation-ids
-         (map car (exec-query conn (insert-into-derivations)))))
+         (map (lambda (result)
+                (string->number (car result)))
+              (exec-query conn (insert-into-derivations)))))
 
     (simple-format
      #t "debug: insert-missing-derivations: updating hash table\n")
@@ -580,7 +582,8 @@ WHERE " criteria ";"))
              conn
              (select-existing-derivations missing-file-names)
              second ;; file_name
-             first))) ;; id
+             (lambda (result)
+               (string->number (first result)))))) ;; id
        (simple-format
         #t "debug: derivation-file-names->vhash: adding ~A entries to the cache\n"
         (vlist-length result-for-missing-file-names))