ci: improve environment detection for protocol-testing builds

[ghost]

Summary

Improves environment detection in the protocol-testing build script to handle differences between local development and CI environments more gracefully.

Changes

• Enhanced platform detection logic in build.rs
• Added fallback paths for toolchain configuration
• Better error handling for missing environment variables

Testing

• Local: cargo nextest run --bin protocol-testing passes
• Should resolve flaky CI builds on different runner images

Context

Following up on recent CI environment updates. This ensures the build script works reliably across stable and nightly toolchains on various platforms.