Handle EINTR error in waitpid call for POSIX

[robo-monk]

Using a debugger such as GDB or CodeLLDB on a program that uses nob.h and depends something like this:

if (!nob_cmd_run(&cmd)) {
    fatal("Failed to run command");
  };

causes the debugging process to detach because of:

[ERROR] could not wait on command (pid 16237): Interrupted system call

This happens when a signal interrupts the waitpid() call, setting errno to EINTR.

This fix adds a retry logic for waitpid() to correctly handle EINTR, ensuring the command wait loop resumes normally instead of treating it as a fatal error.