[bazel] Change cache-silo-key to fix blob fetch issue.

Bazel builds currently fail with `Failed to fetch blobs because they do not exist remotely.`. 

Set a cache-silo-key to start a new cache.

Author: chsigg

utils/bazel/.bazelrc

@@ -211,8 +211,7 @@ build:ci --test_summary=terse
 
 # Attempt to work around intermittent issue while trying to fetch remote blob.
 # See e.g. https://github.com/bazelbuild/bazel/issues/18694.
-build:ci --experimental_remote_cache_eviction_retries=2
-build:ci --experimental_remote_cache_lease_extension=true
+build:ci --remote_default_exec_properties=cache-silo-key=CleverPeafowl
 
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