install_prerequisites.sh: Enforce non-root user execution

Many tools to be installed in the build environment do not require
root permissions. This change enforces the script to be run as a non-root user.

The user will be prompted by sudo to enter the password to allow the
installation of apt packages to proceed.

Signed-off-by: Lokesh B V <Lokesh.BV@Arm.com>
Change-Id: Ib5a4d7473b07f98f1ee1fe61d17a329bc13b3aa4