[ci] Fix mac pipeline (use python 2 in CI scripts)

[krfricke]

Signed-off-by: Kai Fricke kai@anyscale.com

Why are these changes needed?

determine_tests_to_run.py uses python2 on mac, so we need to keep compatibility there

Related issue number

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• I've run scripts/format.sh to lint the changes in this PR.
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