Use 'shell' ansible module in place of 'command'

This is a compound command, piping the output of a call to gpg2 to the input of
a call to grep. This is the type of instruction that requires a shell to interpret, so use
the 'shell' module.

roles/gitian/tasks/gpg.yml

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