Fix path setup for cereal installation script

Author: ZhouXY-PKU

toolchain/scripts/stage4/install_cereal.sh

@@ -117,9 +117,9 @@ if [ "$with_cereal" != "__DONTUSE__" ]; then
     if [ "$with_cereal" != "__SYSTEM__" ]; then
         cat << EOF > "${BUILDDIR}/setup_cereal"
 prepend_path CPATH "$pkg_install_dir/include"
-prepend_path CMAKE_PREFIX_PATH "$pkg_install_dir/include"
-export CPATH="$pkg_install_dir/include":\${CPATH}
-export CMAKE_PREFIX_PATH="$pkg_install_dir/include":\${CMAKE_PREFIX_PATH}
+prepend_path CMAKE_PREFIX_PATH "${pkg_install_dir}/include"
+export CPATH="${pkg_install_dir}/include":\${CPATH}
+export CMAKE_PREFIX_PATH="${pkg_install_dir}/include":\${CMAKE_PREFIX_PATH}
 export CEREAL_ROOT="$pkg_install_dir"
 EOF
     else