feature/clientupdate: windows update should use tailscale.exe update (#19438)

Currently, clientupdate.NewUpdater().Update() is called directly inside tailscaled, which fatals. There is also a failure that doesn't return, causing a panic.

This fix allows us to use the same approach as startAutoUpdate, which is to find tailscale.exe and run tailscale.exe --update, though since it's calling the updater library directly, we get progress messages.

Fixes tailscale/corp#40430s

Signed-off-by: kari-ts <kari@tailscale.com>

feature/clientupdate/clientupdate.go

@@ -163,6 +163,7 @@ func (e *extension) DoSelfUpdate() {
 	})
 	if err != nil {
 		e.pushSelfUpdateProgress(ipnstate.NewUpdateProgress(ipnstate.UpdateFailed, err.Error()))
+		return
 	}
 	err = up.Update()
 	if err != nil {