feat(deploy-dgnum): Update workflow

.forgejo/workflows/deploy-dgnum.yaml

@@ -8,12 +8,16 @@ on:
 
 jobs:
   publish:
-    runs-on: native
+    runs-on: debian-latest
     steps:
       - uses: actions/checkout@v3
         with:
           path: dgnum.eu
 
+      - uses: cachix/install-nix-action@v22
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
       - name: Build website
         run: |
           # Hack to set the cache in the working directory