Use specific mail-robot group to send mail

Allows any user to be able to send mail from the robot account as long
as it is added to the mail-robot group.

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

m/hut/gitea.nix

@@ -29,6 +29,9 @@
     };
   };
 
+  # Allow gitea user to send mail
+  users.users.gitea.extraGroups = [ "mail-robot" ];
+
   services.gitea-actions-runner.instances = {
     runrun = {
       enable = true;