CI: Add job to validate success of matrix

We would like to configure branch protection rules, so that a PR cannot
be merged without CI passing. But configuring which checks need to pass
can be tedious when adding and removing Ruby versions. But by adding one
job that validates the success of the matrix, it becomes simple.

Author: bquorning

.github/workflows/actions.yml

@@ -30,3 +30,18 @@ jobs:
         run: bundle exec rubocop --parallel
       - name: RSpec
         run: bundle exec rspec
+
+  success:
+    name: Checks passing?
+    needs: main
+    if: always()
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          if ${{ needs.main.result == 'success' }}
+          then
+            echo "All checks passed"
+          else
+            echo "Some checks failed"
+            false
+          fi