dialNode picks the destination port from n.DERPPort when non-zero,
falling back to 443 (or 3340 when useHTTPS is false). The proxy path,
dialNodeUsingProxy, hardcoded "443" in the CONNECT target, so a DERP
server reachable only on a custom port was unreachable through
HTTPS_PROXY: the proxy would faithfully tunnel to :443 at the DERP
hostname, and TLS would either fail cert validation or talk to the
wrong service.
Mirror dialNode's port selection so both paths behave the same.
Fixes#19748
Signed-off-by: Martin Zihlmann <martizih@outlook.com>
Martin ZihlmannandBrad Fitzpatrick
4c8c0baf2b