[ci] install freetype on macos for {sysfonts}

.github/workflows/main.yml

       RGL_USE_NULL: true
       # if you use bookdown or blogdown, replace "PKGDOWN" by the respective
       # capitalized term. This also might need to be done in tic.R
-      BUILD_PKGDOWN: ${{ matrix.config.pkgdown }} 
+      BUILD_PKGDOWN: ${{ matrix.config.pkgdown }}
       # macOS >= 10.15.4 linking
       SDKROOT: /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk
 
           # set compiler flags
           mkdir -p ~/.R && echo -e 'CC=ccache clang\nCPP=ccache clang\nCXX=ccache clang++\nCXX11=ccache clang++\nCXX14=ccache clang++\nCXX17=ccache clang++\nCFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.13.sdk\nCCFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.13.sdk\nCXXFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.13.sdk\nCPPFLAGS=-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.13.sdk\nF77=ccache /usr/local/gfortran/bin/gfortran\nFC=ccache /usr/local/gfortran/bin/gfortran' > $HOME/.R/Makevars
 
+      # install freetype for {sysfonts}
+      - name: "[macOS] libfreetype"
+        if: runner.os == 'macOS'
+        run: |
+          brew install freetype
+
       # for some strange Windows reason this step and the next one need to be decoupled
       - name: "[Stage] Prepare"
         run: |