Report Invalid channel 'stable' when execute nixos_search

nix config


```nix

  nixos-mcp = {
    type = "stdio";
    command = "uvx";
    args = ["mcp-nixos"];
  };


```

and report this


```log

nixos-mcp - nixos_search (MCP)(channel: "stable", query: "yazi")
⎿  Error (ERROR): Invalid channel 'stable'. Available channels:

```


