(gha) pin spec tests to use puppet 7.28.0 due to a 7.29.0 bug

See: puppetlabs/puppet#9268

Author: jhoblitt

.github/workflows/rake_checks.yaml

@@ -23,3 +23,6 @@ jobs:
       - run: bundle install
       - name: run rake checks
         run: bundle exec rake
+        env:
+          # https://github.com/puppetlabs/puppet/issues/9268
+          PUPPET_GEM_VERSION: 7.28.0