Stop krew update for release-1.8

[RainbowMango]

What type of PR is this? /kind cleanup

What this PR does / why we need it:

Stop krew update on release-1.8, see #4703 for more details.

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 51.76%. Comparing base (f283bc0) to head (e2a79cd).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.8 #4768 +/- ## =============================================== - Coverage 51.77% 51.76% -0.02% =============================================== Files 243 243 Lines 24117 24117 =============================================== - Hits 12486 12483 -3 - Misses 10948 10950 +2 - Partials 683 684 +1 ``` | [Flag](https://app.codecov.io/gh/karmada-io/karmada/pull/4768/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=karmada-io) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/karmada-io/karmada/pull/4768/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=karmada-io) | `51.76% <ø> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=karmada-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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[karmada-bot]

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