fix: preserve signal-based exit codes (128 + signal_number)

[wolfv]

When a child process is terminated by a signal, return the POSIX-standard exit code (128 + signal_number) instead of falling back to 1.

This fixes incorrect exit codes when processes receive signals like SIGPIPE, SIGTERM, or SIGINT (e.g., yes | head -n 1 now correctly reports 141 for the SIGPIPE-terminated yes process).

Ref: https://github.com/prefix-dev/pixi/issues/5016