gitea: add actions runner and CI/CD deploy workflow

This will avoid me having to run "deploy" myself on my laptop.
All I will need to do is push a commit and it will self-deploy.

services/gitea.nix

@@ -37,6 +37,7 @@
       };
       # only I shall use gitea
       service.DISABLE_REGISTRATION = true;
+      actions.ENABLED = true;
     };
   };