cmd/tailscale: remove dep on clientupdate package if feature is omitted

We already had a featuretag for clientupdate, but the CLI wasn't using
it, making the "minbox" build (minimal combined tailscaled + CLI
build) larger than necessary.

Updates #12614

Change-Id: Idd7546c67dece7078f25b8f2ae9886f58d599002
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/update.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build !ts_omit_clientupdate
+
 package cli
 
 import (
@@ -17,6 +19,14 @@ import (
 	"tailscale.com/version/distro"
 )
 
+func init() {
+	maybeUpdateCmd = func() *ffcli.Command { return updateCmd }
+
+	clientupdateLatestTailscaleVersion.Set(func() (string, error) {
+		return clientupdate.LatestTailscaleVersion(clientupdate.CurrentTrack)
+	})
+}
+
 var updateCmd = &ffcli.Command{
 	Name:       "update",
 	ShortUsage: "tailscale update",