ci: also install modflow-export manually to avoid automatically reintalling pandas < 3

[aleaf]

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[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.08%. Comparing base (186c949) to head (4c6df14).
⚠️ Report is 5 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #205      +/-   ##
===========================================
+ Coverage    86.07%   86.08%   +0.01%     
===========================================
  Files           19       19              
  Lines         4387     4427      +40     
===========================================
+ Hits          3776     3811      +35     
- Misses         611      616       +5     

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