CI: disable KAS_REPO_REF_DIR by default

Having local repo caches is a little fiddly to manage, and by definition
we're running CI inside GitLab which supports mirroring repositories
automatically.

As these mirrors are always available and update automatically, make
Kas reference directories opt-in and instead expect that the site is
either fine with full fetches, or is using KAS_PREMIRRORS.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

.gitlab-ci.yml

@@ -36,7 +36,8 @@ stages:
     KUBERNETES_CPU_REQUEST: $CPU_REQUEST
     KAS_WORK_DIR: $CI_PROJECT_DIR/work
     KAS_BUILD_DIR: $KAS_WORK_DIR/build
-    KAS_REPO_REF_DIR: $CACHE_DIR/repos
+    # Set this in the environment to enable local repository caches
+    KAS_REPO_REF_DIR: ""
     SSTATE_DIR: $CACHE_DIR/sstate
     DL_DIR: $CACHE_DIR/downloads
     BB_LOGCONFIG: $CI_PROJECT_DIR/ci/logging.yml