actions: remove use of set-env command

Ref: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

.github/workflows/tests.yml

@@ -42,7 +42,7 @@ jobs:
         run: |
           curl -fsS -o get-poetry.py https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py
           python get-poetry.py --preview -y
-          echo "::set-env name=PATH::$HOME/.poetry/bin:$PATH"
+          echo "$HOME/.poetry/bin" >> $GITHUB_PATH
 
       - name: Configure poetry
         shell: bash