flake: switch nixpkgs to nixos-unstable, bump to 3ec6363

nixos-unstable dropped nixVersions.nix_2_31, which clan-core still pins
for clan-cli (pkgs/clan-cli/flake-module.nix); override it to pkgs.nix in
the devshell.

flake.lock

@@ -890,16 +890,16 @@
     },
     "nixpkgs_3": {
       "locked": {
-        "lastModified": 1792506237,
-        "narHash": "sha256-Q/8KLB6jNfpaUN1HAedE/u/Et9feE/zYSsutBLKrCFk=",
+        "lastModified": 1792773339,
+        "narHash": "sha256-rZLrzrC6i+jnTAlvPZHogKDulY7WWDEv9hJeHieO7Zw=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "5adc50831c259f87faeb09ddf930d43597197473",
+        "rev": "3ec636362e1816bc2f0888fa0fa2c69f1bc9e504",
         "type": "github"
       },
       "original": {
         "owner": "nixos",
-        "ref": "nixpkgs-unstable",
+        "ref": "nixos-unstable",
         "repo": "nixpkgs",
         "type": "github"
       }

flake.nix

@@ -31,7 +31,7 @@
       inputs.home-manager.follows = "home-manager";
     };
     import-tree.url = "github:vic/import-tree";
-    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
     treefmt-nix = {
       url = "github:numtide/treefmt-nix";
       inputs.nixpkgs.follows = "nixpkgs";

shell.nix

@@ -4,12 +4,14 @@
     inputs.devshell.flakeModule
   ];
   perSystem =
-    { system, ... }:
+    { pkgs, system, ... }:
     {
       devshells.default = {
         devshell = {
           packages = [
-            inputs.clan-core.packages.${system}.clan-cli
+            # clan-core pins `nixVersions.nix_2_31` for a CI perf A/B test;
+            # nixos-unstable has dropped that attribute. Use nixpkgs' default nix.
+            (inputs.clan-core.packages.${system}.clan-cli.override { nix = pkgs.nix; })
           ];
         };
       };