TensorFlow doesn't use bazel-under-test

I ran a downstream build with a custom Bazel to check my flag flips. TensorFlow failed due to a hard-coded max-bazel-version test. However, [judging by the logs](https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/851#bab9da10-9838-4583-9147-7301e0e9ea92) it appears to be fetching its own Bazel version rather than using the one under test.

Is there a local workflow I can use to try building TensorFlow with a custom Bazel (assuming I modify the hardcoded version check)?