ci: refactor (#106)

Signed-off-by: Daijiro Fukuda <[email protected]>

Author: daipom

.github/workflows/linux.yml

@@ -16,15 +16,13 @@ jobs:
           - ubuntu-latest
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
+        bundler-cache: true
     - name: unit testing
       env:
         CI: true
       run: |
-        gem install rake
-        gem install bundler ${{ matrix.ruby == '2.7' && '-v 2.4.22' || '' }}
-        bundle install --jobs 4 --retry 3
         bundle exec rake spec