ci: clang: Prioritise remote Redis cache storage

This commit updates the clang workflow such that ccache only uses remote
Redis cache storage when available.

The purpose of this to reduce the individual runner local disk IOPS
requirement; thereby, reducing the overall load on the SAN.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/clang.yaml

@@ -21,6 +21,7 @@ jobs:
     env:
       CCACHE_DIR: /node-cache/ccache-zephyr
       CCACHE_REMOTE_STORAGE: "redis://cache-*.keydb-cache.svc.cluster.local|shards=1,2,3"
+      CCACHE_REMOTE_ONLY: "true"
       LLVM_TOOLCHAIN_PATH: /usr/lib/llvm-16
       COMMIT_RANGE: ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }}
       BASE_REF: ${{ github.base_ref }}