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.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";