install_prerequisites.sh: install poetry via offical installer

TF-A added ‘poetry’ as a mandatory build dependency.
Updated prerequisites to include the official installation method for ‘poetry’.

[1] https://github.com/ARM-software/arm-trusted-firmware/commit/793f72c06ca1c2782f800c9f20980ca6b7870072



Signed-off-by: Lokesh B V <Lokesh.BV@Arm.com>
Change-Id: I6a957abd7b24a1113fe907d4d01dc48191b8030d