Use nix eval instead of dagger for nix runner version extraction

Dagger CLI needs a container runtime (Docker/containerd) to start its
engine, which the bare nix runner doesn't have. Use nix eval directly
instead — it's already available and more appropriate for a nix host.

Reverts the dagger flake input since it's not usable here.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nixos/ringtail/configuration.nix

@@ -1,4 +1,4 @@
-{ config, pkgs, lib, dagger-pkg, ... }:
+{ config, pkgs, lib, ... }:
 
 let
   # Libraries needed by mise-compiled runtimes (python-build, etc.)
@@ -500,7 +500,7 @@ in
       tokenFile = "/etc/forgejo-runner/token.env";
       labels = [ "nix-container-builder:host" ];
       hostPackages = with pkgs; [
-        bash coreutils curl dagger-pkg gawk gitMinimal gnused jq nodejs wget
+        bash coreutils curl gawk gitMinimal gnused jq nodejs wget
         nix skopeo
       ];
       settings = {