ci: Update Woodpecker to use Nix

.woodpecker/build-site.yml

@@ -2,20 +2,16 @@ when:
   branch: "master"
 
 steps:
-  - name: Install Node packages
-    image: node:18.14.2-slim
+  - name: Build site
+    image: nixos/nix:2.24.11
+    privileged: true
     commands:
-      - npm ci
-  - name: Build Hugo site
-    image: peaceiris/hugo:v0.111.3-full
-    environment:
-      HUGO_ENV: production
-      HUGO_ENVIRONMENT: production
-    commands:
-      - hugo --minify
+      - echo 'experimental-features = flakes nix-command' >> /etc/nix/nix.conf
+      - echo 'sandbox = true' >> /etc/nix/nix.conf
+      - nix build
   - name: Deploy to pages
     image: codeberg.org/xfix/plugin-codeberg-pages-deploy:1
     settings:
-      folder: public
+      folder: result
       ssh_key:
         from_secret: ssh_key