ci: Fix Woodpecker symlink deployment issue

Woodpecker wasn't able to deploy, because 'result' from a Nix flake is a
symlink to the Nix store, which isn't copied over to a non-Nix
deployment container like plugin-codeberg-pages-deploy. Trying to
troubleshoot deploy steps in particular is hard because, not only do you
not want to have the test stage doing random deployments, but Woodpecker
CLI also features a nice bug where the use of `from_secret` actually
causes the entire pipeline to not run.

Anyway, remove excess permissions that are no longer needed, create a
proper copy step, and fix the deployment.

.woodpecker/build-site.yml

@@ -1,17 +1,17 @@
 when:
-  branch: "master"
+  branch: master
+  event: push
 
 steps:
   - name: Build site
     image: nixos/nix:2.24.11
-    privileged: true
     commands:
       - echo 'experimental-features = flakes nix-command' >> /etc/nix/nix.conf
-      - echo 'sandbox = true' >> /etc/nix/nix.conf
       - nix build
+      - mkdir public && cp -av result/* public
   - name: Deploy to pages
     image: codeberg.org/xfix/plugin-codeberg-pages-deploy:1
     settings:
-      folder: result
+      folder: public
       ssh_key:
         from_secret: ssh_key