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.

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; })
           ];
         };
       };