Restore aarch64-unknown-linux-gnu release builds

[humancto]

Problem

aarch64-linux builds were temporarily removed from the release workflow due to GLIBC version mismatch in the cross Docker container. This means no ARM Linux binary (Raspberry Pi, AWS Graviton, etc.).

Root Cause

cross Docker image has older glibc than what the build scripts need. Needs a Cross.toml with a custom Docker image that has compatible glibc.

Solution

1. Create Cross.toml with custom Docker image for aarch64-unknown-linux-gnu
2. Re-add target to release.yml build matrix
3. Re-add aarch64-linux to Homebrew formula
4. Verify binary runs on ARM Linux