Define top level Permission for fossa.yml (FOSSA) workflow

[aditya7302]

trafficstars

What type of PR is this?

/kind feature

What this PR does / why we need it:

Which issue(s) this PR fixes: Part of #5048

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

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Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5080   +/-   ##
=========================================
  Coverage          ?   28.21%           
=========================================
  Files             ?      632           
  Lines             ?    43568           
  Branches          ?        0           
=========================================
  Hits              ?    12294           
  Misses            ?    30378           
  Partials          ?      896           

Flag	Coverage Δ
unittests	28.21% <ø> (?)

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[zhzhuang-zju]

/lgtm cc @RainbowMango

[karmada-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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