Include helm/rustfs/README.md in the uploaded helm-package artifact so
publish-helm-package updates the root README.md in rustfs/helm.
The build-helm-package job copies helm/README.md into helm/rustfs/
before running helm package, but upload-artifact previously uploaded
only helm/rustfs/*.tgz. Because both files share the helm/rustfs/
parent directory, actions/upload-artifact places both the chart
archive and README.md at the artifact root, and download-artifact
extracts README.md into the root of rustfs/helm alongside the tarball.