Fix env variable in CI detection

[paleite]

Summary

• fix environment variable name to ESLINT_PLUGIN_DIFF_COMMIT
• update CI tests accordingly

Testing

• npm test

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https://chatgpt.com/codex/tasks/task_e_686395b00074832abceea0c14eb3cf97

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.26%. Comparing base (bf3f207) to head (d44dbff).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files           5        5           
  Lines         173      173           
  Branches       49       49           
=======================================
  Hits          170      170           
  Misses          3        3           

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