sgi/install_prerequisites.sh: replace python-pip with python3-pip

python-pip package is required for installation of cmake. Starting from
Ubuntu 20.04, `python-pip` package is not available as part of the apt
repository. Replace it with `python3-pip` as python3 is the default
version in Ubuntu 20.04. This is also backward compatible with Ubuntu
18.04.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I5c5d58c84e9caf1e46945eb1e732466514d6e46b