brew cask install -> brew install --cask

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
           sudo apt-get install -y xvfb herbstluftwm
         elif [ "${{ matrix.os }}" = "macos-latest" ]; then
           brew install coreutils
-          brew cask install xquartz
+          brew install --cask xquartz
         fi
 
     - name: Set up Python