Use nix unstable

modules/base/nix/default.nix

@@ -52,6 +52,7 @@ in
     };
 
     nix = {
+      package = pkgs.nixVersions.unstable;
       gc = {
         automatic = true;
         dates = "hourly";

shells/nixos-config.nix

@@ -17,7 +17,7 @@
   packages = [
     pkgs.agenix
     # Use nixos-rebuild from flake, since it might be patched
-    pkgs.nixos-rebuild
+    (pkgs.nixos-rebuild.override { nix = pkgs.nixVersions.unstable; })
     pkgs.nixpkgs-fmt
   ];
 }