nix: add gc

configuration.nix

@@ -86,6 +86,13 @@
   nix = {
     # optimize the store
     optimise.automatic = true;
+
+    # garbage collection
+    gc = {
+      automatic = true;
+      dates = "weekly";
+      options = "--delete-older-than 7d";
+    };
   };
 
   hardware.intelgpu.driver = "xe";