guix-install.sh: Please the shellcheck linter.

guix-install.sh: Please the shellcheck linter.

* etc/guix-install.sh <FUNCNAME>: Explicitly refer to the first item of the
FUNCNAME array.
(ROOT_HOME): Replace variable by ~root directly; manually expanding it via
echo was not necessary.
(chk_gpg_keyring): Use an if branch for the exit to avoid a warning about
expression precedence.
(chk_term) <ansi_term>: Remove unused variable.
(guix_get_bin) <wget, gpg>: Test the commands directly.  Use an array for the
wget arguments, which can then be properly expanded.
(sys_create_store): Disable SC1090 for the source command, as we don't care
about following the sourced script.
<_msg>: Reuse the GUIX_PROFILE variable in the message.
(sys_create_build_user) <getent>: Test the command directly.