feat: verify GitHub release source attestations

[wolfv]

Summary

• discover immutable GitHub release attestations through the repository attestations API, filtered by the downloaded asset's SHA-256 and the release predicate
• verify release bundles using GitHub's Sigstore trust root, timestamp evidence, release-service certificate identity, and signed pkg:github/owner/repo@version subject
• reject dynamically generated GitHub source archives, whose bytes are not covered by the release attestation
• document the uploaded source.tar.gz workflow and add a runnable uv recipe example

Motivation

GitHub release attestation download IDs are unstable and cannot be derived from a source URL. The repository attestations API avoids scraping those URLs. GitHub's generated "Source code" archives are only represented by the release commit SHA-1 and are not cryptographically bound to the downloaded archive bytes, while uploaded release assets are represented by SHA-256 and can be verified safely.

Testing

• cargo test -p rattler_build_source_cache --features sigstore,rustls
• cargo check -p rattler_build_core --features sigstore,rustls
• cargo test -p rattler_build_recipe --test source_parsing_test test_parse_uv_attestation_example
• end-to-end ignored network test against astral-sh/uv 0.12.1 source.tar.gz