build(flake): switch to stable by default

Switch to using the stable version of nixpkgs but still use unstable
where it's needed.

system/nixos/nixedo/configuration.nix

@@ -114,7 +114,7 @@ in
     isNormalUser = true;
     description = "Oliver Davies";
     extraGroups = [ "docker" "networkmanager" "wheel" ];
-    packages = with pkgs; [];
+    packages = with pkgs; [ ];
   };
 
   security.sudo.wheelNeedsPassword = false;