chore: remove all occurrences of github.event

[rix0rrr]

We have code scanners at Amazon that complain about all occurrences of ${{ github.event.* }} inside GitHub Actions run blocks, even if they're safe.

In this case, base.ref and issue.number cannot actually be exploited by attackers, but the scanner still trips over it.

Route these through environment variables to appease the scanner.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.59%. Comparing base (2d462a4) to head (5d15c36).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1722   +/-   ##
=======================================
  Coverage   89.59%   89.59%           
=======================================
  Files          77       77           
  Lines       11758    11758           
  Branches     1651     1651           
=======================================
  Hits        10534    10534           
  Misses       1195     1195           
  Partials       29       29           

Flag	Coverage Δ
suite.unit	89.59% <ø> (ø)

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