Switch setup-uv action to immutable releases

[EmmanuelNiyonshuti]

Updated astral-sh/setup-uv from @v7 to a pinned commit hash for v8.1.0.

Description

Motivation and Context

The setup-uv maintainers moved to an immutable release model starting with v8. They no longer support floating major tags (like @v8), which means automation tools won't see new updates if we stay on @v7.

This change improves CI security by pinning to a specific hash and ensures the project is aligned with the new versioning scheme.

How Has This Been Tested?

This is a CI-only change. I believe the workflows should trigger and run as usual using the new pinned version of the action.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Let me know if this is a change you'd want to roll out across the other repos or something else. thanks!

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks

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_{Comparing EmmanuelNiyonshuti:chore/uv-immutable-updates (d553432) with develop (d31209c)}