Fix rebuild helper external target release path

[cbusillo]

Summary

• preserve the repo release-bin path when rebuilding with an external CARGO_TARGET_DIR
• restore PATH verification for release-bin symlinks by comparing resolved targets
• resolve relative CARGO_TARGET_DIR values the same way Cargo does from the current working directory

Validation

• ./build-fast.sh
• CARGO_TARGET_DIR=$(mktemp -d /tmp/code-rebuild-target.XXXXXX) scripts/local/rebuild-path-code.sh
• scripts/local/rebuild-path-code.sh