Run Bazel test suite on Linux ARM64 platform

Make LabelValidator compatible with external callers.

Failed in 2m 30s
:ubuntu: Ubuntu 20.04 LTS ARM64curl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1739452518 -o bazelci.py && python3.8 bazelci.py runner --task=ubuntu2004_arm64 --http_config=https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/pipelines/bazel-linux-arm64.yml?1739452517
Waited 6s
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Ran in 1m 46s
Total Job Run Time: 2m 25s