Update conda install workflow

[znicholls]

Description

Working workflow: https://github.com/openscm/pandas-openscm/actions/runs/17266614633/job/49000144546

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.29%. Comparing base (9f27b32) to head (6932f28).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
=======================================
  Coverage   97.29%   97.29%           
=======================================
  Files          31       31           
  Lines        1479     1479           
  Branches      164      164           
=======================================
  Hits         1439     1439           
  Misses         23       23           
  Partials       17       17           

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[znicholls]

All working: https://github.com/openscm/pandas-openscm/actions/runs/17266824943