CI: update conda setup

[martinfleis]

Update conda recipe to avoid failure due to https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

[codecov-io]

Codecov Report

Merging #151 (cd2f87e) into master (41a57ff) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   77.61%   77.61%           
=======================================
  Files          25       25           
  Lines        3274     3274           
=======================================
  Hits         2541     2541           
  Misses        733      733           

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