Temporarily test C++ on GB300

Author: bdice

.github/workflows/pr.yaml

@@ -277,6 +277,8 @@ jobs:
     if: fromJSON(needs.changed-files.outputs.changed_file_groups).test_cpp
     with:
       build_type: pull-request
+      matrix_type: nightly
+      matrix_filter: map(select(.GPU == "gb300"))
       script: ci/test_cpp.sh
   conda-cpp-debug-tests:
     needs: [checks, changed-files]