fix: update to new GH token method

Author: jeffabailey

.github/workflows/mdbook_deploy.yml

@@ -51,7 +51,7 @@ jobs:
         run: mdbook build
 
       - name: Set up GitHub token
-        run: echo "::set-env name=GITHUB_TOKEN::${{ secrets.GITHUB_TOKEN }}"
+        run: echo "GITHUB_TOKEN=${{ secrets.GITHUB_TOKEN }}" >> $GITHUB_ENV
 
       - name: Deploy to GitHub Pages
         uses: peaceiris/actions-gh-pages@v3