Use setup homebrew action

Ubuntu 18.04 runner is deprecated and linuxbrew-wrapper is no longer
exist in new runner. See actions/runner-images#6283

Ref:
- https://github.com/Homebrew/actions/tree/master/setup-homebrew
- https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2004-Readme.md

.github/workflows/github-pages.yml

@@ -10,7 +10,8 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
-      - run: sudo apt install linuxbrew-wrapper
+      - name: Set up Homebrew
+        uses: Homebrew/actions/setup-homebrew@master
       - run: brew install coreutils
       #- run: cargo install mdbook
       - run: chmod 777 -R ./
@@ -27,4 +28,4 @@ jobs:
         uses: peaceiris/actions-gh-pages@v3
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: ./book
+          publish_dir: ./book