See https://discourse.phabricator-community.org/t/arc-call-conduit-does-not-work-on-windows/4146.

Currently, when arc does a read from stdin, it sets stdin nonblocking so the read will be interrupted by signals. We want users to be able to ^C out of arc call-conduit (with no arguments), etc.

This fails on Windows because stdin can not be made nonblocking on Windows. The effort is also pointless on Windows because we currently can't handle signals there anyway (see also T13549).