Mock instead of setting real env var

[hugovk]

Follow up to #107, mock the env var instead of setting it directly, so as not to affect other tests.

[codecov[bot]]

Codecov Report

Merging #109 (548f9eb) into main (e576831) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
+ Coverage   95.30%   95.33%   +0.03%     
==========================================
  Files           9        9              
  Lines         383      386       +3     
==========================================
+ Hits          365      368       +3     
  Misses         18       18              

Flag	Coverage Δ
macos-latest	95.33% <100.00%> (+0.03%)	:arrow_up:
ubuntu-latest	95.33% <100.00%> (+0.03%)	:arrow_up:
windows-latest	95.33% <100.00%> (+0.03%)	:arrow_up:

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Impacted Files	Coverage Δ
tests/test_norwegianblue.py	100.00% <100.00%> (ø)

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