[rubygems/rubygems] Get a CI matrix configured in default Github Actions generated config

Even if it only has one entry at the moment, it makes it easier to add
new entries by doing it this way.

https://github.com/rubygems/rubygems/commit/46232fe265

Co-authored-by: NeimadTL <damientalbot26@gmail.com>

lib/bundler/templates/newgem/github/workflows/main.yml.tt

@@ -9,12 +9,18 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        ruby:
+          - <%= RUBY_VERSION %>
+
     steps:
     - uses: actions/checkout@v2
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with:
-        ruby-version: <%= RUBY_VERSION %>
+        ruby-version: ${{ matrix.ruby }}
         bundler-cache: true
     - name: Run the default task
       run: bundle exec rake