guix-install.sh: Be POSIX-compliant.

* etc/guix-install.sh (guix_get_bin_list): Call grep with an extended
regular expression instead of a non-POSIX Perl regular expression.
(sys_create_store): Remove ‘--warning=no-timestamp’ argument to tar.

Signed-off-by: Tobias Geerinckx-Rice <me@tobias.gr>