Added separate rubocop workflow.

.github/workflows/rubocop.yml

@@ -0,0 +1,17 @@
+name: lint
+
+on: [push, pull_request]
+
+jobs:
+  lint:
+    name: RuboCop
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3.5'
+          bundler-cache: true
+      - name: Run RuboCop
+        run: bundle exec rubocop