clientupdate: add explicit Track to Arguments (#10548)

Instead of overloading the Version field, add an explicit Track field.

This fixes a bug where passing a track name in `args.Version` would keep
the track name in `updater.Version` and pass it down the code path to
commands like `apt-get install`. Now, `updater.Version` should always be
a version (or empty string).

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

cmd/tailscale/cli/update.go

@@ -59,12 +59,9 @@ func runUpdate(ctx context.Context, args []string) error {
 	if updateArgs.version != "" && updateArgs.track != "" {
 		return errors.New("cannot specify both --version and --track")
 	}
-	ver := updateArgs.version
-	if updateArgs.track != "" {
-		ver = updateArgs.track
-	}
 	err := clientupdate.Update(clientupdate.Arguments{
-		Version: ver,
+		Version: updateArgs.version,
+		Track:   updateArgs.track,
 		Logf:    func(f string, a ...any) { printf(f+"\n", a...) },
 		Stdout:  Stdout,
 		Stderr:  Stderr,