Fix gitea user to allow sending email

In order to send email, the gitea user needs to be in the mail-robot
group.

Fixes: #220
Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/tent/gitea.nix

@@ -27,4 +27,7 @@
       };
     };
   };
+
+  # Allow gitea user to send mail
+  users.users.gitea.extraGroups = [ "mail-robot" ];
 }