Pin conda used in testing

[dstansby]

It's a long shot, but maybe helps with #683 🤷

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (6c0ea0f) to head (173fbe3).
Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #684   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          62       62           
  Lines        2752     2752           
=======================================
  Hits         2750     2750           
  Misses          2        2           

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[dstansby]

No luck 😢