Add olderl-1 to macos test

          - {os: macos-latest,   r: 'oldrel-1'}

Author: aphalo

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

@@ -21,6 +21,7 @@ jobs:
       matrix:
         config:
           - {os: macos-latest,   r: 'release'}
+          - {os: macos-latest,   r: 'oldrel-1'}
           - {os: windows-latest, r: 'release'}
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}