[release-1.8] :bug: Checksum file location in ARTIFACTS dir

[knative-prow-robot]

This is an automated cherry-pick of #235

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cardil, knative-prow-robot

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/knative-sandbox/kn-plugin-event/blob/release-1.8/OWNERS)~~ [cardil] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[codecov[bot]]

Codecov Report

Merging #236 (a6161d4) into release-1.8 (adeb7e9) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff              @@
##           release-1.8     #236   +/-   ##
============================================
  Coverage        67.97%   67.97%           
============================================
  Files               35       35           
  Lines             1121     1121           
============================================
  Hits               762      762           
  Misses             299      299           
  Partials            60       60           

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