fix: Lean build failure on macOS

flake.lock

@@ -38,9 +38,7 @@
         "flake-utils": "flake-utils",
         "lean4-mode": "lean4-mode",
         "nix": "nix",
-        "nixpkgs": [
+        "nixpkgs": "nixpkgs_2",
-          "nixpkgs"
-        ],
         "nixpkgs-old": "nixpkgs-old"
       },
       "locked": {
@@ -195,6 +193,22 @@
       }
     },
     "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1711715736,
+        "narHash": "sha256-9slQ609YqT9bT/MNX9+5k5jltL9zgpn36DpFB7TkttM=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "807c549feabce7eddbf259dbdcec9e0600a0660d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "nixpkgs_3": {
       "locked": {
         "lastModified": 1697456312,
         "narHash": "sha256-roiSnrqb5r+ehnKCauPLugoU8S36KgmWraHgRqVYndo=",
@@ -215,7 +229,7 @@
         "flake-parts": "flake-parts",
         "lean": "lean",
         "lspec": "lspec",
-        "nixpkgs": "nixpkgs_2"
+        "nixpkgs": "nixpkgs_3"
       }
     }
   },

flake.nix

@@ -6,7 +6,6 @@
     flake-parts.url = "github:hercules-ci/flake-parts";
     lean = {
       url = "github:leanprover/lean4?ref=b4caee80a3dfc5c9619d88b16c40cc3db90da4e2";
-      inputs.nixpkgs.follows = "nixpkgs";
     };
     lspec = {
       url = "github:lurk-lab/LSpec?ref=3388be5a1d1390594a74ec469fd54a5d84ff6114";