scripts/build_utils: write settings to /root/.pbuilderrc
according to pbuilder(8)
> ${HOME}/.pbuilderrc
> The personal configuration file for pbuilder, which overrides settings set in other configuration files.
>
> Note that ${HOME} is usually /root (if you are running pbuilder through sudo).
since we always call `pbuilder` via `sudo`, we should overwrite
/root/.pbuilderrc with $HOME/.pbuilderrc.