ci: pin uv cache key to pyproject.toml

The matrix job in commit 7aa4989 left astral-sh/setup-uv@v3 with its
default cache fingerprint of "**/uv.lock". This repo is a library
and intentionally does not commit uv.lock (commit a2df706), so the
glob matches nothing and the action errors out.

Switch the fingerprint to pyproject.toml, the actual source of truth
for dependencies. Cache hits remain across PRs that don't touch
dependencies; cache invalidates on dependency changes.

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