archlinux: Handle reads through new install.sh

This not only fixes the issue where set -x would cause the read
line to be altered, but also separates user input from what the
install scripts actually do.

.archlinux/install-scripts/install.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+set -e
+
+echo -n "Enter a hostname for this machine: "
+read HOSTNAME
+
+echo -n "Enter a username for the local account with sudo rights: "
+read USERNAME
+
+env HOSTNAME="$HOSTNAME" ./003-configure
+env USERNAME="$USERNAME" ./004-postinstall