CI: rely on setup-ruby to install (#264)

...and use a newer actions/checkout (to avoid any GitHub Actions warning about old stuff used)

Author: olleolleolle

.github/workflows/ci.yml

@@ -36,17 +36,13 @@ jobs:
           notify_when: "failure"
         env:
           SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v4
       - name: Install package dependencies
         run: "[ -e $APT_DEPS ] || sudo apt-get install -y --no-install-recommends $APT_DEPS"
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{matrix.ruby}}
-      - name: Install latest bundler
-        run: |
-          gem install bundler --no-document
-      - name: Bundle install
-        run: bundle install --jobs 4 --retry 3
+          bundler-cache: true # 'bundle install' and cache gems
       - name: Run all tests
         run: script/ci