preinst and postinst scripts aren't being saved to the cache.

[gtjoseph]

There are two issues in install_and_cache_pkgs.sh preventing preinst and postinst scripts from being included in the package tarballs...

     tar -cf "${cache_filepath}" -C / --verbatim-files-from --files-from <(
      { dpkg -L "${package_name}" &&
        get_install_script_filepath "" "${package_name}" "preinst" &&
        get_install_script_filepath "" "${package_name}" "postinst" ; } |

1. Since && is being used between the commands that lists the files, if there isn't a preinst script, a postinst script isn't even checked for.
2. get_install_script_filepath has an empty root file path so it will never return any files.

PR forthcoming.