clientupdate: don't link distsign on platforms that don't download

Updates tailscale/corp#20099

Change-Id: Ie3b782379b19d5f7890a8d3a378096b4f3e8a612
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

clientupdate/clientupdate_not_downloads.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !((linux && !android) || windows)
+
+package clientupdate
+
+func (up *Updater) downloadURLToFile(pathSrc, fileDst string) (ret error) {
+	panic("unreachable")
+}