Add rubocop group to Gemfile for use in CI

Since c1e7268c83 we install the latest
version of RuboCop in our GitHub Actions workflow for speed, but this
sacrifices reproducibility; the results will change whenever RuboCop
publishes a new version. Instead we can add a new group to our Gemfile
that just contains the dependencies necessary to run RuboCop, and skip
installing everything else in CI.

Unfortunately it's not possible to tell Bundler to only install gems
from a single group, so we have to tell it not to install every other
group instead.

.github/workflows/rubocop.yml

@@ -12,8 +12,17 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: 2.7
-    - name: Install RuboCop gems
+    - 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: |
-        gem install --no-document rubocop rubocop-performance rubocop-rails
+        bundle config path vendor/bundle
+        bundle config set without 'default doc job cable storage ujs test db'
+        bundle install --jobs 4 --retry 3
     - name: Run RuboCop
-      run: rubocop --parallel
+      run: bundle exec rubocop --parallel

Gemfile

@@ -28,9 +28,11 @@ gem "uglifier", ">= 1.3.0", require: false
 # Explicitly avoid 1.x that doesn't support Ruby 2.4+
 gem "json", ">= 2.0.0"
 
-gem "rubocop", ">= 0.47", require: false
-gem "rubocop-performance", require: false
-gem "rubocop-rails", require: false
+group :rubocop do
+  gem "rubocop", ">= 0.47", require: false
+  gem "rubocop-performance", require: false
+  gem "rubocop-rails", require: false
+end
 
 group :doc do
   gem "sdoc", "~> 1.1"