Build liboxen once in test PyPI workflow

[malcolmgreaves]

Refactors the Test PyPI publish workflow so that liboxen is
only built once per-platform (OS + architecture). The crate is
reused for each individual pyO3 + Python version build.

[malcolmgreaves]

Inspecting changes here: https://github.com/Oxen-AI/Oxen/actions/runs/22242233517

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