change how to handle the cache setting

plugin.sh

@@ -21,10 +21,10 @@ DOCKERJSON
 DOCKERFILE=${PLUGIN_DOCKERFILE:-Dockerfile}
 CONTEXT=${PLUGIN_CONTEXT:-$PWD}
 LOG=${PLUGIN_LOG:-info}
-case "${PLUGIN_CACHE:-}" in
-  true) CACHE="true" ;;
-     *) CACHE="false" ;;
-esac
+
+if [[ "${PLUGIN_CACHE:-}" == "true" ]]; then
+    CACHE="--cache=true"
+fi
 
 if [[ -n "${PLUGIN_BUILD_ARGS:-}" ]]; then
     BUILD_ARGS=$(echo "${PLUGIN_BUILD_ARGS}" | tr ',' '\n' | while read build_arg; do echo "--build-arg=${build_arg}"; done)
@@ -39,6 +39,6 @@ fi
 /kaniko/executor -v ${LOG} \
     --context=${CONTEXT} \
     --dockerfile=${DOCKERFILE} \
-    --cache=${CACHE} \
     ${DESTINATIONS} \
+    ${CACHE:-} \
     ${BUILD_ARGS:-}