trying to fix macosx build issue

* there is a problem installing nat on the github macosx hosts, but can't see errors to figure out what
* rgl seems the most likely culprit so trying to set NULL

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

@@ -20,7 +20,7 @@ jobs:
       fail-fast: false
       matrix:
         config:
-          - {os: macos-latest,   r: 'release'}
+          - {os: macos-latest,   r: 'release', rglusenull: "true"}
           - {os: windows-latest, r: 'release'}
           - {os: ubuntu-latest,   r: 'devel', http-user-agent: 'release'}
           - {os: ubuntu-latest,   r: 'release'}
@@ -29,6 +29,7 @@ jobs:
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
       R_KEEP_PKG_SOURCE: yes
+      RGL_USE_NULL: ${{ matrix.config.rglusenull }}
 
     steps:
       - uses: actions/checkout@v4