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 ✅

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

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Additional details and impacted files

@@               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	Coverage Δ
unittests	51.76% <ø> (-0.02%)	⬇️

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