remove nixpkgs-unstable

the current version of woodpecker-server in stable now has the features this was original used for, not other usages of unstable are in tree left so removing it.

flake.lock

@@ -68,27 +68,10 @@
         "type": "github"
       }
     },
-    "nixpkgs-unstable": {
-      "locked": {
-        "lastModified": 1731265036,
-        "narHash": "sha256-e5I+glVZwQvLT6WIeMFi0Mk+N/jkYauZ31ir2NRZcf8=",
-        "owner": "nixos",
-        "repo": "nixpkgs",
-        "rev": "8aed22ecd71e5b67e5299efae8b9dc580dec711c",
-        "type": "github"
-      },
-      "original": {
-        "owner": "nixos",
-        "ref": "nixos-unstable-small",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "root": {
       "inputs": {
         "nixos-generators": "nixos-generators",
         "nixpkgs": "nixpkgs",
-        "nixpkgs-unstable": "nixpkgs-unstable",
         "sops-nix": "sops-nix"
       }
     },