nix: Only keep outputs and drvs on full systems

modules/default.nix

@@ -111,7 +111,7 @@ in
         # On-the-fly optimisation of nix store
         autoOptimiseStore = true;
         # Keep output of derivations with gc root
-        extraOptions = ''
+        extraOptions = lib.optionalString config.sbruder.full ''
           keep-outputs = true
           keep-derivations = true
         '';