use clang on ubuntu to keep CMTK happy

Author: jefferis

.github/workflows/R-CMD-check.yaml

@@ -20,11 +20,11 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - {os: macos-latest,   r: 'release'}
-          - {os: windows-latest, r: 'release'}
-          - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
-          - {os: ubuntu-latest,   r: 'release', codecov: 'true'}
-          - {os: ubuntu-latest,   r: 'oldrel-1'}
+          - {os: macos-latest,    r: 'release'}
+          - {os: windows-latest,  r: 'release'}
+          - {os: ubuntu-latest, cc: clang, r: 'devel', http-user-agent: 'release'}
+          - {os: ubuntu-latest, cc: clang, r: 'release', codecov: 'true'}
+          - {os: ubuntu-latest, cc: clang, r: 'oldrel-1'}
 
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}