prober,derp/derphttp: make dev-mode DERP probes work without TLS (#14347)

Make dev-mode DERP probes work without TLS. Properly dial port `3340`
when not using HTTPS when dialing nodes in `derphttp_client`. Skip
verifying TLS state in `newConn` if we are not running a prober.

Updates tailscale/corp#24635

Signed-off-by: Percy Wegmann <percy@tailscale.com>
Co-authored-by: Percy Wegmann <percy@tailscale.com>

derp/derphttp/derphttp_client.go

@@ -757,6 +757,9 @@ func (c *Client) dialNode(ctx context.Context, n *tailcfg.DERPNode) (net.Conn, e
 			}
 			dst := cmp.Or(dstPrimary, n.HostName)
 			port := "443"
+			if !c.useHTTPS() {
+				port = "3340"
+			}
 			if n.DERPPort != 0 {
 				port = fmt.Sprint(n.DERPPort)
 			}