ci: Run jobs in a static workspace path

Default workspace may not be the same for Kubernetes GitLab runners.
Hence, build and test job may get executed in different directories.
ctest needs to have build artefacts in same directory path to find the
tests. To get the workspace same, execute the jobs in a pre-defined
workspace by setting the GIT_CLONE_PATH

Signed-off-by: Saheer Babu <saheer.babu@arm.com>