Fix broken GitHub Action (#16)

due to a [security vulnerability in GitHub Actions](https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/), the use of `set-env` was deactivated.
switched to [environment files](https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#environment-files) to avoid this issue.

.github/workflows/release.yml

@@ -23,7 +23,7 @@ jobs:
         pip install -r requirements.txt
         python setup.py sdist bdist_wheel
         cd dist
-        echo "::set-env name=WHEEL::$(ls *.whl)"
+        echo "WHEEL=`ls *.whl`" >> $GITHUB_ENV
       # create a new draft release (so we only trigger the deployment pipeline after the draft has been published)
     - name: Create Release
       id: create_release