docker: update Shrinkwrap image to install LLVM/Clang v20

This patch replaces the manual tarball-based installation
of the aarch64 LLVM toolchain with an automated setup using
the official apt.llvm.org repository.

This update is required to keep the Shrinkwrap Docker
environment aligned with the Hafnium project's transition to
Clang v20 and ensure consistent toolchain behavior across
both projects.

Legacy environment variables and download logic have been
removed from "build.sh" accordingly.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>