Update main.yml

Author: mcbarton

.github/workflows/main.yml

@@ -329,7 +329,7 @@ jobs:
                          --contents notebooks/smallpt.ipynb \
                          --contents notebooks/images/marie.png \
                          --contents notebooks/audio/audio.wav &
-          if [[ "${os}" == "macos"* ]]; then
+          if [[ "${{ matrix.os }}" == "macos"* ]]; then
             brew install coreutils
             export PATH="$HOMEBREW_PREFIX/opt/coreutils/libexec/gnubin:$PATH"
           fi