use actions checkout to checkout branch

.github/workflows/gh-pages.yml

@@ -20,8 +20,11 @@ jobs:
       - name: build
         run: cmake --build build -t doc
       - name: gh-pages
-        run: git checkout gh-pages
-      - name: cleanup
-        run: ls -lah && rm -r !("build")
+        uses: actions/checkout@master
+        with:
+          ref: "gh-pages"
+          clean: true
+      # - name: cleanup
+      #   run: ls -lah && rm -r !("build")
       - name: commit
         run: mv ./build/html . && rm -rf ./build && ls -lah