ci: codecov: Use Redis remote storage for ccache

This commit updates the codecov workflow to, when available, use Redis
remote storage backend for the ccache compilation cache data.

The Redis cache server is hosted in the Kubernetes cluster in which the
zephyr-runner pods run -- the Redis remote storage backend will be ignored
if the server is unavailable.

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

.github/workflows/codecov.yaml

@@ -31,6 +31,7 @@ jobs:
             normalized: 'unit_testing'
     env:
       CCACHE_DIR: /node-cache/ccache-zephyr
+      CCACHE_REMOTE_STORAGE: "redis://cache-*.keydb-cache.svc.cluster.local|shards=1,2,3"
     steps:
       - name: Apply container owner mismatch workaround
         run: |