fix(ci): override target-cpu=native that breaks macOS CI

The .cargo/config.toml has target-cpu=native which causes ring crate
to fail on GitHub runners due to CPU feature detection mismatches.
Set RUSTFLAGS="" in CI to override.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

.github/workflows/ci.yml

@@ -9,6 +9,8 @@ on:
 env:
   CARGO_TERM_COLOR: always
   RUST_BACKTRACE: 1
+  # Override target-cpu=native from .cargo/config.toml (breaks CI runners)
+  RUSTFLAGS: ""
 
 jobs:
   build: