Replace --remote_default_platform_properties with --remote_default_exec_properties (#2409)

The old flag has been deleted in
https://github.com/bazelbuild/bazel/commit/f6356d6cdb9a36d06e5638ea1ef3d83033631293
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index ba7166b..c78f186 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1870,7 +1870,7 @@
     flags += [
         f"--remote_timeout={remote_timeout}",
         "--remote_max_connections=200",
-        '--remote_default_platform_properties=properties:{name:"cache-silo-key" value:"%s"}'
+        '--remote_default_exec_properties=cache-silo-key=%s'
         % platform_cache_digest.hexdigest(),
         "--remote_download_toplevel",
     ]