build(ci): Build linux-arm64 on GitHub's native arm64 runners

[notheotherben]

Switches the linux-arm64 build from cross to GitHub's native arm64 runners (ubuntu-24.04-arm, free for public repositories).

What changed

• build matrix: the arm64 Linux entry now runs on ubuntu-24.04-arm and builds aarch64-unknown-linux-gnu natively.
• Removed the cargo-binstall/cross install step, along with the comment documenting why it had to be ordered before the cache restore (binstall would otherwise see cross as installed and skip materialising the binary).
• With every entry now building with cargo, the builder matrix key is gone.

No extra setup is needed for the target: the dependency graph is pure Rust apart from aws-lc-sys, which builds against the runner's own toolchain the same way it does on the amd64 entry.

Why

The arm64 build was the only reason the workflow installed cross at all, and keeping that install correct required careful ordering against Swatinem/rust-cache. On a native arm64 runner the target builds with exactly the same steps as every other entry in the matrix.

Verification

CI on this PR builds linux-arm64 on the native runner; the release-only steps (artifact upload, Docker packaging) are unchanged and consume the same artifact name as before.

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Generated by Claude Code

[Copilot]

Pull request overview

This pull request simplifies the CI build workflow by switching the Linux ARM64 build from cross to GitHub Actions’ native ARM64 runner (ubuntu-24.04-arm), making the ARM64 build follow the same native cargo build flow as the other matrix entries.

Changes:

• Migrated the linux-arm64 matrix entry to run natively on ubuntu-24.04-arm and build aarch64-unknown-linux-gnu with cargo.
• Removed the builder matrix key and the conditional cross installation step (and its cache-ordering workaround comment).
• Standardized the build step to always run cargo build --release --target ....

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.83%. Comparing base (cb1c640) to head (5aa7d41).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #662   +/-   ##
=======================================
  Coverage   76.83%   76.83%           
=======================================
  Files          31       31           
  Lines        2612     2612           
=======================================
  Hits         2007     2007           
  Misses        605      605           

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