fix: try nix runner.

.forgejo/workflows/default.yml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   build:
-    runs-on: native
+    runs-on: nix
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
       - run: nix flake check

modules/servers/per-server/freyja/services/forgejo-runner.nix

@@ -13,7 +13,7 @@
       # tokenFile should be in format TOKEN=<secret>, since it's EnvironmentFile for systemd
       tokenFile = config.age.secrets.forgejo-runner-token.path;
       labels = [
-        "nix-stable:docker://nixos/nix:2.32.0"
+        "nix:docker://forgejo-runner-nix"
         ## optionally provide native execution on the host:
         "native:host"
       ];