ci: coverage for system ruby with package libraries

Author: flavorjones

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
       fail-fast: false
       matrix:
         image: ["ubuntu"]
-        sys: ["enable"]
+        sys: ["enable", "disable"]
     runs-on: ubuntu-latest
     container:
       image: ghcr.io/sparklemotion/nokogiri-test:${{matrix.image}}