message: release: add npm mirror url into release note
  For example:
diff: @@ -204,6 +204,8 @@ jobs:
  uses: softprops/action-gh-release@v1
  with:
  name: "Nydus Image Service ${{ env.tag }}"
  + body: |
  + Mirror (update in 10 min): https://registry.npmmirror.com/binary.html?path=nydus/${{ env.tag }}/
  generate_release_notes: true
  files: |
  ${{ env.tarballs }}
  