Fix tty detection for msys2's `/dev/ptmx`
Our "true negative" detection assumes that if at least one std handle is a Windows console then no other handle will be a msys2 tty pipe. This turns out to be a faulty assumption in the case of redirection to `/dev/ptmx` in an msys2 shell. Maybe this is an msys2 bug but in any case we should try to make it work.
An alternative to this would be to replace the "true negative" detection with an attempt to detect if we're in an msys environment (e.g. by sniffing environment variables) but that seems like it'd be flaky too.
Fixes#119658
67d093682c