test macos fix

Author: zbjornson

prebuild/macOS/preinstall.sh

@@ -8,6 +8,8 @@ rm -f /usr/local/bin/pydoc3 || :
 rm -f /usr/local/bin/python3 || :
 rm -f /usr/local/bin/python3-config || :
 
+xcode-select --install
+
 # two or more of these packages have python3 as a dependency, and --overwrite
 # doesn't work to make them ignore conflicts. For now just ignore errors, yolo.
 brew install --force pkg-config cairo pango librsvg giflib || :