Enable Rubocop in GitHub Actions

.github/workflows/ci.yml

@@ -84,3 +84,19 @@ jobs:
         if: ${{ success() || failure() }}
         env:
           CC_TEST_REPORTER_ID: ${{ secrets.CC_TEST_REPORTER_ID }}
+
+  lint:
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      - name: Setup Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+          bundler-cache: true
+
+      - name: Run rubocop
+        run: bundle exec rubocop