Disable nix garbage collector in tent

Reviewed-by: Aleix Boné <abonerib@bsc.es>
Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

m/tent/configuration.nix

@@ -67,4 +67,7 @@
     wantedBy = [ "multi-user.target" ];
     serviceConfig.ExecStart = script;
   };
+
+  # disable automatic garbage collector
+  nix.gc.automatic = lib.mkForce false;
 }