gpg: Fix honoring --cert-digest-algo when recreating a cert

* g10/sign.c (update_keysig_packet): Override original signature's
digest algo in hashed data and for hash computation.

g10/sign.c

@@ -1604,6 +1604,8 @@ update_keysig_packet( PKT_signature **ret_sig,
     /* create a new signature packet */
     sig = copy_signature (NULL, orig_sig);
 
+    sig->digest_algo=digest_algo;
+
     /* We need to create a new timestamp so that new sig expiration
        calculations are done correctly... */
     sig->timestamp=make_timestamp();