We can just stringify the normal nixpkgs flake.

flake.lock

@@ -121,23 +121,6 @@
         "type": "github"
       }
     },
-    "nixpkgs-nonflake": {
-      "flake": false,
-      "locked": {
-        "lastModified": 1679437018,
-        "narHash": "sha256-vOuiDPLHSEo/7NkiWtxpHpHgoXoNmrm+wkXZ6a072Fc=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "19cf008bb18e47b6e3b4e16e32a9a4bdd4b45f7e",
-        "type": "github"
-      },
-      "original": {
-        "owner": "NixOS",
-        "ref": "nixos-unstable",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "nixpkgs-stable": {
       "locked": {
         "lastModified": 1678872516,
@@ -186,7 +169,6 @@
         "devshell": "devshell",
         "flake-utils": "flake-utils",
         "nixpkgs": "nixpkgs",
-        "nixpkgs-nonflake": "nixpkgs-nonflake",
         "pre-commit-hooks": "pre-commit-hooks",
         "rust-overlay": "rust-overlay",
         "treefmt-nix": "treefmt-nix"

flake.nix

@@ -1,12 +1,6 @@
 {
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    # We re-use the systemd lib from NixOS, this input allows to import the needed modules.
-    # TODO: is there a better way to do this?
-    nixpkgs-nonflake = {
-      url = "github:NixOS/nixpkgs/nixos-unstable";
-      flake = false;
-    };
     flake-utils.url = "github:numtide/flake-utils";
     devshell = {
       url = "github:numtide/devshell";
@@ -47,7 +41,6 @@
   outputs =
     { self
     , nixpkgs
-    , nixpkgs-nonflake
     , flake-utils
     , rust-overlay
     , crane
@@ -189,7 +182,7 @@
     {
       lib = import ./nix/lib.nix {
         inherit nixpkgs self;
-        nixosModules = "${nixpkgs-nonflake}/nixos";
+        nixosModules = "${nixpkgs}/nixos";
       };
 
       systemConfigs.default = self.lib.makeSystemConfig {