.github/workflows/release: make sure GH_TOKEN is set for upload

[croissanne]

I thought the GH_TOKEN was set automatically 🙄

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.17%. Comparing base (9bbf85c) to head (fa998f5).
Report is 4 commits behind head on main.

@@           Coverage Diff           @@
##             main    #2824   +/-   ##
=======================================
  Coverage   83.17%   83.17%           
=======================================
  Files         190      190           
  Lines       22233    22233           
  Branches     2174     2174           
=======================================
  Hits        18492    18492           
  Misses       3719     3719           
  Partials       22       22           

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