feat: uv_version to pin uv

[jmelahman]

Passes along the version value to setup-uv.

It can be considered best practice to pin this version a la supply-chain concerns, but more practically, this action hits the Github API if this is unset and we're getting rate-limited,

Run astral-sh/setup-uv@eb1897b8dc4b5d5bfe39a428a8f2304605e0983c
(node:1557) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
(Use `node --trace-deprecation ...` to show where the warning was created)
Trying to find version for uv in: /home/runner/_work/onyx/onyx/uv.toml
Could not find file: /home/runner/_work/onyx/onyx/uv.toml
Trying to find version for uv in: /home/runner/_work/onyx/onyx/pyproject.toml
Could not determine uv version from uv.toml or pyproject.toml. Falling back to latest.
Getting latest version from GitHub API...
Error: Github API request failed while getting latest release. Check the GitHub status page for outages. Try again later.
Error: API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID DFFE:3AC19:A0F67B:2C82286:695D8D30 and timestamp 2026-01-06 22:31:12 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/rest/overview/rate-limits-for-the-rest-api