* etc/guix-install.sh (ROOT_HOME): New variable.
(sys_create_store, sys_enable_guix_daemon, sys_authorize_build_farms):
Use ROOT_HOME instead of ~root or the HOME environment variable.
* doc/guix.texi (Binary Installation): Instead of assuming that ~ and
$HOME refer to root's directory simply because commands are being run
as root, explicilty refer to it via ~root.
Fixes: <https://bugs.gnu.org/30728>