[release-1.7] Fix release script version calculation

[knative-prow-robot]

This is an automated cherry-pick of #1737

Fix release script version calculation

[dsimansk]

/approve /lgtm

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[codecov[bot]]

Codecov Report

Base: 79.93% // Head: 79.93% // No change to project coverage :thumbsup:

Coverage data is based on head (24af349) compared to base (6d86bf7). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.7 #1741 +/- ## ============================================ Coverage 79.93% 79.93% ============================================ Files 174 174 Lines 13532 13532 ============================================ Hits 10817 10817 Misses 1978 1978 Partials 737 737 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=knative). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=knative)

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