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.go

@@ -27,7 +27,6 @@ import (
 	"strconv"
 	"strings"
 
-	"tailscale.com/clientupdate/distsign"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/cmpver"
 	"tailscale.com/version"
@@ -778,14 +777,6 @@ func (up *Updater) cleanupOldDownloads(glob string) {
 	}
 }
 
-func (up *Updater) downloadURLToFile(pathSrc, fileDst string) (ret error) {
-	c, err := distsign.NewClient(up.Logf, up.PkgsAddr)
-	if err != nil {
-		return err
-	}
-	return c.Download(context.Background(), pathSrc, fileDst)
-}
-
 func (up *Updater) updateFreeBSD() (err error) {
 	if up.Version != "" {
 		return errors.New("installing a specific version on FreeBSD is not supported")

clientupdate/clientupdate_downloads.go

@@ -0,0 +1,20 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (linux && !android) || windows
+
+package clientupdate
+
+import (
+	"context"
+
+	"tailscale.com/clientupdate/distsign"
+)
+
+func (up *Updater) downloadURLToFile(pathSrc, fileDst string) (ret error) {
+	c, err := distsign.NewClient(up.Logf, up.PkgsAddr)
+	if err != nil {
+		return err
+	}
+	return c.Download(context.Background(), pathSrc, fileDst)
+}

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")
+}