tiup update --self always downloads the latest binary even if already up-to-date

[mjonss]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

tiup update --self
tiup update --self

2. What did you expect to see?
   Only download the latest version if not already using it.

3. What did you see instead?
   Every time tiup update --self is run, it will download the latest version, even if already running it.

4. What version of TiUP are you using (tiup --version)?

1.9.6 tiup
Go Version: go1.17.10
Git Ref: v1.9.6
GitHash: 28965259aecd0a10ae72b69fc59121f6b4edd276