[lava, child] Use CI_REGISTRY_IMAGE in image paths

The original use of CI_REGISTRY/CI_PROJECT_PATH results in invalid
registry access paths as the CI_PROJECT_PATH could contain uppercase
letters that are not allowed in registry paths.  The change switches to
the correctly formatted CI_REGISTRY_IMAGE variable in image references.

Changelog: bug
Signed-off-by: Drew Reed <Drew.Reed@arm.com>