Run both Ubuntu variants

Author: eddelbuettel

.github/workflows/ci.yaml

@@ -17,7 +17,7 @@ jobs:
           - { name: container, os: ubuntu-latest, container: rocker/r2u4ci }
           - { name: macos,     os: macos-latest,  openmp: yes }
           #- { name: macos,     os: macos-latest,  openmp: no }
-          #- { name: ubuntu,    os: ubuntu-latest }
+          - { name: ubuntu,    os: ubuntu-latest }
 
 
     runs-on: ${{ matrix.os }}