* ci: invoke pip via python -m in s3tests clone steps
The self-hosted runner reaches the Clone s3-tests step with the
setup-python toolcache interpreter on PATH but no pip shim, so the
bare 'pip install' exits 127. python -m pip resolves through the
interpreter that is already on PATH, matching the helm_ci workflow.
* ci: invoke pip via python3 -m in s3tests clone steps
The adjacent fix_s3_tests_bucket_conflicts step already runs python3, so
python3 is the interpreter guaranteed on the self-hosted PATH; a missing
python alias would fail the same way the missing pip shim did.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
---------
Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
