fix(linter): rename error variables to follow ST1012 convention

[jijo-OO7]

Fix ST1012 linter staticcheck issues (Fixes #4420)

Changes

• Re-enable ST1012 check in .golangci.yaml and fix error variable naming conventions:
• Update all related references

Testing

golangci-lint run ./... passes without ST1012 errors

[knative-prow]

Hi @jijo-OO7. Thanks for your PR.

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[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jijo-OO7
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.77%. Comparing base (98368cf) to head (e8251f5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4597   +/-   ##
=======================================
  Coverage   28.77%   28.77%           
=======================================
  Files         294      294           
  Lines       16169    16169           
=======================================
  Hits         4653     4653           
  Misses      11063    11063           
  Partials      453      453           

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[jijo-OO7]

Hi @creydr ,
When you have a moment, could you please review this PR? Thank you!