Switched to environment files

Author: squidfunk

.github/workflows/documentation.yml

@@ -54,7 +54,7 @@ jobs:
           GH_TOKEN: ${{ secrets.GH_TOKEN }}
         run: |
           git clone --depth 1 https://${GH_TOKEN}@github.com/squidfunk/mkdocs-material-insiders.git
-          echo "::set-env name=THEME_DIR::mkdocs-material-insiders/material"
+          echo "THEME_DIR=mkdocs-material-insiders/material" >> $GITHUB_ENV
 
       - name: Deploy documentation
         env: