.oniro-ci: use repo --reference conditionally

The reference is provided only when repo is used with runner persistent
storage. This is not a major change, just a clean-up of the existing
logic.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>

.oniro-ci/build-generic.yaml

@@ -35,8 +35,8 @@
         echo "CI_ONIRO_MANIFEST_URL: $CI_ONIRO_MANIFEST_URL"
         echo "CI_ONIRO_MANIFEST_NAME: $CI_ONIRO_MANIFEST_NAME"
         echo "CI_ONIRO_MANIFEST_BRANCH: $CI_ONIRO_MANIFEST_BRANCH"
-        repo init --reference \
-           "$CI_ONIRO_RUNNER_PERSISTENT_STORAGE"/pub/git-repo-mirrors/"$CI_ONIRO_MANIFEST_MIRROR" \
+        repo init \
+           $(test -n "${CI_ONIRO_RUNNER_PERSISTENT_STORAGE:-}" && echo --reference "$CI_ONIRO_RUNNER_PERSISTENT_STORAGE"/pub/git-repo-mirrors/"$CI_ONIRO_MANIFEST_MIRROR") \
            --manifest-url "$CI_ONIRO_MANIFEST_URL" \
            --manifest-name "$CI_ONIRO_MANIFEST_NAME" \
            --manifest-branch "$CI_ONIRO_MANIFEST_BRANCH"