chore: use ssh instead of https for nil inside flake.nix

ssh scheme will use ssh key instead of username/password for fetching private repo

flake.nix

@@ -25,7 +25,7 @@
     };
     nil-cluster = {
       type = "git";  # 'git' is required here, for 'github' we cannot compute cluster version via git history
-      url = "https://github.com/NilFoundation/nil";
+      url = "ssh://git@github.com/NilFoundation/nil";
       inputs = {
         nixpkgs.follows = "nixpkgs";
         flake-utils.follows = "flake-utils";