Use BUNDLE_ONLY: rubocop suported since Bundler 2.3.19

Bundler 2.3.19 can install gems of the specific group now.

Refer to
https://github.com/rubygems/rubygems/blob/master/bundler/CHANGELOG.md#2319-july-27-2022
https://github.com/rubygems/rubygems/pull/5759
https://github.com/rails/rails/pull/38836

.github/workflows/rubocop.yml

@@ -9,7 +9,7 @@ jobs:
   build:
     runs-on: ubuntu-latest
     env:
-      BUNDLE_WITHOUT: default doc job cable storage ujs test db
+      BUNDLE_ONLY: rubocop
 
     steps:
     - uses: actions/checkout@v3