timeout: replace nix by rustix

[oech3]

I guess this is done as same as #12326 .

[gabrielhnf]

Timeout currently uses signal/sigaction, since rustix doesn't expose signal-related syscalls, should those stay on nix while the rest migrates to rustix, or fall back to libc directly?

[oech3]

OOops... Does sigaction -> libc increase unsafe?