fix: build with uv instead of pip in the uv-run PSR step

[irfanuddinahmad]

#435's uv-based workaround got past the GitPython crash (confirmed: this run correctly computed "The next version is: 4.1.1!") but hit a new failure at the build step: https://github.com/openedx/event-tracking/actions/runs/33071634441

PSR's build_command (pyproject.toml) shells out to python -m pip install --upgrade build, but uv's ephemeral venvs deliberately don't bundle pip the way a stdlib venv/virtualenv does.

Verified locally through the full real uvx-wrapped PSR execution, not just uv build standalone: produces the correct dist/*.tar.gz/*.whl artifacts.

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