kas-container: Fix mounting of custom KAS_REPO_REF_DIR

kas-container support for KAS_REPO_REF_DIR was broken. The path provided
by the env variable was the path on the local machine, not in the
container.

Signed-off-by: Rotem Bar <rotemb@hailo.ai>
[Jan: massaged commit log]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas-container

@@ -372,7 +372,7 @@ fi
 if [ -n "${KAS_REPO_REF_DIR}" ]; then
 	set -- "$@" \
 		-v "$(readlink -f "${KAS_REPO_REF_DIR}")":/repo-ref:ro \
-		-e KAS_REPO_REF_DIR="${KAS_REPO_REF_DIR}"
+		-e KAS_REPO_REF_DIR=/repo-ref
 fi
 
 for var in TERM KAS_DISTRO KAS_MACHINE KAS_TARGET KAS_TASK \