message: action: add shasum file for released assets
  User can use this shasum file to verify the validity of
  the downloaded nydus release tarball files.
diff: @@ -148,11 +148,17 @@ jobs:
  chmod +x nydus-static/*
  tar cf - nydus-static | gzip > ${tarball}
  echo "tarball=${tarball}" >> $GITHUB_ENV
  +
  + shasum="$tarball.sha256sum"
  + sha256sum $tarball > $shasum
  + echo "tarball_shasum=${shasum}" >> $GITHUB_ENV
  - name: store-artifacts
  uses: actions/upload-artifact@v2
  with:
  name: nydus-release-tarball
  - path: ${{ env.tarball }}
  + path: |
  + ${{ env.tarball }}
  + ${{ env.tarball_shasum }}
  # use a seperate job for darwin because github action if: condition cannot handle && properly.
  prepare-tarball-darwin:
  @@ -175,11 +181,17 @@ jobs:
  chmod +x nydus-static/*
  tar cf - nydus-static | gzip > ${tarball}
  echo "tarball=${tarball}" >> $GITHUB_ENV
  +
  + shasum="$tarball.sha256sum"
  + sha256sum $tarball > $shasum
  + echo "tarball_shasum=${shasum}" >> $GITHUB_ENV
  - name: store-artifacts
  uses: actions/upload-artifact@v2
  with:
  name: nydus-release-tarball
  - path: ${{ env.tarball }}
  + path: |
  + ${{ env.tarball }}
  + ${{ env.tarball_shasum }}
  create-release:
  runs-on: ubuntu-latest
  