Clean up the shared-state-cache

Allows cleaning share state cache by declaring CI_ONIRO_CLEAN_SSTATE variable, manually, via a schedule pipeline or globally.

Related to: eclipse-wg/oniro-wg/products-services-oniro-wg/it-services-oniro-wg/pipelines-architecture-oniro-wg#58 (closed)

Signed-off-by: sebastien.heurtematte sebastien.heurtematte@eclipse-foundation.org

assigned to @stefanschmidt

mentioned in merge request !332

changed title from ci: clean up the shared-state-cache to Clean up the shared-state-cache