ignore errors on 'Check that the secret key exists' task

If the grep for gpg_key_id fails to match we will have an exit code of 1 which by
default will cause the task to fail. According to the README, using a gpg key is considered optional. So we don't want the build to halt in this case.

roles/gitian/tasks/gpg.yml

@@ -1,6 +1,7 @@
 ---
 - name: Check that the secret key exists.
   local_action: "shell gpg2 --list-secret-keys --with-colons | grep {{ gpg_key_id }}"
+  ignore_errors: true
   register: gpg_list_keys_result
   environment:
     GNUPGHOME: "{{ lookup('env', 'HOME') }}/.gnupg"