Restore override key with a comment (#9298)

Co-authored-by: Ashwin Srinath <shwina@users.noreply.github.com>

Author: shwina

ci/matrix.yaml

@@ -20,6 +20,9 @@ workflows:
   #   - { jobs: ['run_gpu'], project: 'target', ctk: ['12.X', '13.X'], cxx: ['gcc', 'clang'], gpu: 'rtx2080',
   #       args: '--preset libcudacxx --lit-tests "cuda/utility/basic_any.pass.cpp"' }
   #
+  # IMPORTANT: Do NOT delete or remove the `override:` key below, even when it is empty.
+  override:
+
   pull_request:
     # Old CTK: Oldest/newest supported host compilers:
     - {jobs: ['build'], std: 'minmax', ctk: '12.0', cxx: ['gcc7',  'gcc12', 'clang14',            'msvc2019', 'msvc14.39']}