workflows/release-tasks: Upload release notes as an artifact

[tstellar]

Summary

This make sure the docs are always available and can be manually uploaded
if a later step fails.

.github/workflows/release-tasks.yml

steps:
         run: |
           ./llvm/utils/release/./github-upload-release.py --token ${{ github.token }} --release ${{ steps.validate-tag.outputs.release-version }} create
 
       - name: Build Documentation
         run: |
           ./llvm/utils/release/build-docs.sh -srcdir llvm
           ./llvm/utils/release/github-upload-release.py --token ${{ github.token }} --release ${{ steps.validate-tag.outputs.release-version }} upload --files *doxygen*.tar.xz
 
+      - name: Create Release Notes Artifact
+        uses: actions/download-artifact@v1
+        with:
+          name: release-notes
+          path: docs-build/html-export/
+
       - name: Clone www-releases
         if: ${{ !contains(steps.validate-tag.outputs.release-version, 'rc') }}
         uses: actions/checkout@v3
         with:
           repository: ${{ github.repository_owner }}/www-releases
           ref: main
           fetch-depth: 0
           path: www-releases