On domain-joined Windows devices the primary search domain (the one the device is joined to)
always takes precedence over other search domains. This breaks MagicDNS when we are the primary
resolver on the device (see #18712). To work around this Windows behavior, we should write MagicDNS
host names the hosts file just as we do when we're not the primary resolver.
This commit does exactly that.
Fixes#18712
Signed-off-by: Nick Khyl <nickk@tailscale.com>