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"
       }