ci: use toolchain from docker image

Similarly to [1], update PATH to use clang toolchain sourced from docker
image for the following Hafnium jobs:

hafnium-code-coverage-fvp
hafnium-hypervisor-tf-spmc-test-fvp
hafnium-spmc-test-fvp

[1] https://review.trustedfirmware.org/c/ci/hafnium-ci-scripts/+/21723

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I5237fc3a4387e8e2a2270d28c592fa99f42c56cc
3 files changed