Add dagger to nix-container-builder runner (#234)

## Summary
- Add `dagger` to `hostPackages` for the ringtail nix-container-builder runner
- Needed for `dagger call nix-version` fallback in the nix build workflow (authentik)
- `hostPackages` is scoped to the runner's systemd unit PATH, not system-wide
- Marks `install-dagger-on-nix-runner` Mikado card complete

## Deployment and Testing
- [ ] Merge, then `mise run provision-ringtail`
- [ ] `mise run container-build-and-release authentik` to verify nix build succeeds

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/234

nixos/ringtail/configuration.nix

@@ -500,7 +500,7 @@ in
       tokenFile = "/etc/forgejo-runner/token.env";
       labels = [ "nix-container-builder:host" ];
       hostPackages = with pkgs; [
-        bash coreutils curl gawk gitMinimal gnused jq nodejs wget
+        bash coreutils curl dagger gawk gitMinimal gnused jq nodejs wget
         nix skopeo
       ];
       settings = {