CI: create build/ before compiling the server; drop artifact upload

upload-artifact@v4 is unsupported on Gitea (GHES error) and failed the
release job after a successful build; the release asset is the
artifact. dart compile exe doesn't create the output directory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -30,12 +30,6 @@ jobs:
       - name: Build tarball
         run: tool/package_linux.sh
 
-      - name: Upload tarball artifact
-        uses: actions/upload-artifact@v4
-        with:
-          name: linux-tarball
-          path: dist/*.tar.gz
-
       - name: Publish release
         if: startsWith(github.ref, 'refs/tags/v')
         env: