Move the delete-unreferenced-derivations advisory lock

To better prevent two processes running at the same time.

guix-data-service/data-deletion.scm

@@ -561,10 +561,6 @@ WHERE NOT EXISTS (
                       (with-postgresql-transaction
                        conn
                        (lambda (conn)
-                         (obtain-advisory-transaction-lock
-                          conn
-                          'delete-unreferenced-derivations)
-
                          (exec-query
                           conn
                           "
@@ -585,6 +581,10 @@ SET CONSTRAINTS derivations_by_output_details_set_derivation_id_fkey DEFERRED")
       (lambda ()
         (with-thread-postgresql-connection
          (lambda (conn)
+           (obtain-advisory-transaction-lock
+            conn
+            'delete-unreferenced-derivations)
+
            (let loop ((total-deleted 0))
              (let ((batch-deleted-count (delete-batch conn)))
                (if (eq? 0 batch-deleted-count)