.github: adapt to new artifact action behavior

No more artifact/ subdirectory with upload/download-artifact v4,
artifacts are already in the working directory.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/build.yml

@@ -102,7 +102,7 @@ jobs:
           prerelease: true
           tag: "latest"
           token: ${{ secrets.GITHUB_TOKEN }}
-          artifacts: "artifact/*"
+          artifacts: "*.tar.gz"
       - name: Summary
         run: |
           cat <<EOF >> $GITHUB_STEP_SUMMARY