Switch build/site to build-site in order to keep symlinks working (#211)

The jekyll build keeps top-level without expanding them. So the site works, but the publish copy failed because some symlinks to files that aren't visible through the site (like CONTRIBUTING.md, versus the site's CONTRIBUTING.html) didn't work.

.github/workflows/publish-docs.yaml

@@ -36,4 +36,4 @@ jobs:
         run: make build
         working-directory: website/jekyll
       - name: Publish to www.carbon-lang.dev
-        run: gsutil cp -R website/jekyll/build/site/* gs://www.carbon-lang.dev/
+        run: gsutil cp -R website/jekyll/build-site/* gs://www.carbon-lang.dev/