Move rubocop to a separate CI job

We use a very old version of rubocop to support
ruby 2.0, however that version is not compatible with
ruby-head.

Either way there's not point running rubocop on every
ruby version a distinct CI job is more efficient.

Author: byroot

.github/workflows/build.yml

@@ -61,4 +61,4 @@ jobs:
         run: echo 'DB=${{ matrix.db }}' >> $GITHUB_ENV
       - run: sudo echo "127.0.0.1 mysql2gem.example.com" | sudo tee -a /etc/hosts
       - run: bash ci/setup.sh
-      - run: bundle exec rake
+      - run: bundle exec rake spec

.github/workflows/rubocop.yml

@@ -0,0 +1,27 @@
+name: RuboCop
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Ruby 2.4
+      uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: 2.4
+    - name: Cache gems
+      uses: actions/cache@v1
+      with:
+        path: vendor/bundle
+        key: ${{ runner.os }}-rubocop-${{ hashFiles('**/Gemfile.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-rubocop-
+    - name: Install gems
+      run: |
+        bundle config path vendor/bundle
+        bundle install --jobs 4 --retry 3
+    - name: Run RuboCop
+      run: bundle exec rubocop