Fix Linux tmux segfault in user shell lookup

[etraut-openai]

Replace the Unix shell lookup path in codex-rs/core/src/shell.rs to use
libc::getpwuid_r() instead of libc::getpwuid() when resolving the current
user's shell.

Why:

• getpwuid() can return pointers into libc-managed shared storage
• on the musl static Linux build, concurrent callers can race on that storage
• this matches the crash pattern reported in tmux/Linux sessions with parallel
  shell activity

Refs:

• Fixes Crashed (segmentation fault, exited 139) inside tmux on Linux, introduced after v0.110.0 #13842

[qtnx]

Thank you for this. Please consider adding it to the next release. I implemented the same approach in my local fork, and Codex has been running for over two days without any issues in the same environment, setup, and prompt.