CI: Use bundler-cache

I didn't change the Lint job, since it gave me weird issues in CI, not
locally, with gems not in this project showing up as being required, and
not existing.

.github/workflows/test.yml

@@ -79,10 +79,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-      - name: Bundle
-        run: |
-          gem install bundler
-          bundle install --jobs 4 --retry 3
+          bundler-cache: true # 'bundle install' and cache
         env:
           BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile