Use rtassert! instead of assert! from the child process after fork() in std::sys::unix::process::Command::spawn()

`assert!` panics on failure, which is not signal-safe.

library/std/src/sys/unix/process/process_unix.rs

@@ -67,7 +67,7 @@ impl Command {
                     // pipe I/O up to PIPE_BUF bytes should be atomic, and then
                     // we want to be sure we *don't* run at_exit destructors as
                     // we're being torn down regardless
-                    assert!(output.write(&bytes).is_ok());
+                    rtassert!(output.write(&bytes).is_ok());
                     libc::_exit(1)
                 }
                 n => n,