cmd/serve: don't convert localhost to 127.0.0.1

This is not valid in many situations, specifically when running a local astro site that listens on localhost, but ignores 127.0.0.1

Fixes: https://github.com/tailscale/tailscale/issues/12201

Signed-off-by: Josh McKinney <joshka@users.noreply.github.com>

ipn/serve.go

@@ -538,7 +538,7 @@ func ExpandProxyTargetValue(target string, supportedSchemes []string, defaultSch
 		return "", fmt.Errorf("invalid port %q", u.Port())
 	}
 
-	u.Host = fmt.Sprintf("%s:%d", host, port)
+	u.Host = fmt.Sprintf("%s:%d", u.Hostname(), port)
 
 	return u.String(), nil
 }