flake.nix: Unfollow nixpkgs in flake-parts

This may speed up autocompletion since the default lib is a small
path.

(It shouldn't matter, but Nix isn't as lazy as it will be yet, so
here we go.)

(cherry picked from commit 90709ff3c690275dfb6b8fce50cb0282a52e1971)

flake.lock

@@ -74,9 +74,7 @@
     },
     "flake-parts": {
       "inputs": {
-        "nixpkgs-lib": [
-          "nixpkgs"
-        ]
+        "nixpkgs-lib": "nixpkgs-lib"
       },
       "locked": {
         "lastModified": 1722555600,
@@ -282,6 +280,18 @@
         "type": "github"
       }
     },
+    "nixpkgs-lib": {
+      "locked": {
+        "lastModified": 1724440177,
+        "narHash": "sha256-uFf2QeW7eAHlYXuDktm9c25OxOyCoUOQmh5SZ9amE5Q=",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/a5d394176e64ab29c852d03346c1fc9b0b7d33eb.tar.gz"
+      },
+      "original": {
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/a5d394176e64ab29c852d03346c1fc9b0b7d33eb.tar.gz"
+      }
+    },
     "nixpkgs-regression": {
       "locked": {
         "lastModified": 1643052045,

flake.nix

@@ -3,7 +3,6 @@
 
   inputs = {
     flake-parts.url = "github:hercules-ci/flake-parts";
-    flake-parts.inputs.nixpkgs-lib.follows = "nixpkgs";
     nix.url = "github:NixOS/nix";
     nix.inputs.nixpkgs.follows = "nixpkgs";
     nix-cargo-integration.url = "github:yusdacra/nix-cargo-integration";