Implement a workaround for macOS/Bazel flakes.

Bazel has a 2 minute timeout for their internal `xcrun` call, which can be exceeded on our github runners about 5% of the time.  This leads to flakes and opaque errors, but is a one-time cost.  Subsequent xcruns finish in seconds, so we can just do an initial call w/o a timeout before running Bazel.

With this change our total flake rate drops from ~30% to nearly 0% for our full suite of tests

See https://github.com/bazelbuild/bazel/issues/17437 for background.

PiperOrigin-RevId: 509944178