Don't delete old bazel bin in kokoro build and ensure KOKORO_ARTIFACTS_DIR is set
Don't delete old bazel bin in kokoro build and ensure KOKORO_ARTIFACTS_DIR is set

This creates problems if the bazel binary isn't there or if KOKORO_ARTIFACTS_DIR is unset, respectively. Both of these happens when I'm fiddling with these from our new custom VMs.

With these changes, I'm able to run the Kokoro script from our custom VM images and it should still work on the old ones, so allows a seamless changeover.

PiperOrigin-RevId: 285501255

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