fix: remove unsafe set_var from test

[hstove-stacks]

I was bothered by my IDE warning me about this use of "unsafe" code, so I went to refactor it. Turns out this ENV var just isn't used, so it can be removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.87%. Comparing base (7ff75a2) to head (1030ca1).
⚠️ Report is 10 commits behind head on develop.

❌ Your project status has failed because the head coverage (70.87%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6734      +/-   ##
===========================================
+ Coverage    70.05%   70.87%   +0.81%     
===========================================
  Files          578      578              
  Lines       358373   358370       -3     
===========================================
+ Hits        251064   253987    +2923     
+ Misses      107309   104383    -2926     

Files with missing lines	Coverage Δ
stacks-node/src/tests/signer/v0.rs	49.59% <ø> (+19.71%)	⬆️

... and 386 files with indirect coverage changes

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[francesco-stacks]

LGTM, I just double checked that it wasn't used anywhere in the code