Use separate workflow for rubocop

Author: horgh

.github/workflows/rubocop.yml

@@ -0,0 +1,12 @@
+name: Run rubocop
+on: [push, pull_request]
+jobs:
+  rubocop:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 2.7
+      - run: bundle install
+      - run: bundle exec rake -t rubocop

.github/workflows/test.yml

@@ -25,7 +25,5 @@ jobs:
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.version }}
-      - name: ruby version
-        run: ruby -v
       - run: bundle install
-      - run: bundle exec rake -t
+      - run: bundle exec rake -t test