Fix deprecation warning in GitHub Actions.

See GitHub's post about switching away from add-path:
https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Author: QuLogic

.github/workflows/reviewdog.yml

@@ -22,7 +22,7 @@ jobs:
           curl -sfL \
             https://github.com/reviewdog/reviewdog/raw/master/install.sh | \
               sh -s -- -b $HOME/bin
-          echo ::add-path::$HOME/bin
+          echo $HOME/bin >> $GITHUB_PATH
 
       - name: Run flake8
         env: