CI: reduce workflow permissions

[pjonsson]

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📚 Documentation preview 📚: https://datacube-ows--1240.org.readthedocs.build/en/1240/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.38%. Comparing base (5544143) to head (0d4ce8f).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1240   +/-   ##
========================================
  Coverage    93.38%   93.38%           
========================================
  Files           53       53           
  Lines         7125     7125           
========================================
  Hits          6654     6654           
  Misses         471      471           

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