use withenv instead of persistent ENV variables

[t-bltg]

Leaving persistent ENV variables is a bad habbit.
If we really need those set because of the way GR works by design, we should clear them after usage.

[codecov]

Codecov Report

Base: 80.84% // Head: 80.87% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (0ebf099) compared to base (c7e3191).
Patch coverage: 58.82% of modified lines in pull request are covered.

❗ Current head 0ebf099 differs from pull request most recent head 9c2f000. Consider uploading reports for the commit 9c2f000 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4402      +/-   ##
==========================================
+ Coverage   80.84%   80.87%   +0.03%     
==========================================
  Files          30       30              
  Lines        7448     7445       -3     
==========================================
  Hits         6021     6021              
+ Misses       1427     1424       -3     

Impacted Files	Coverage Δ
src/backends/gr.jl	89.30% <58.82%> (+0.21%)	⬆️

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[t-bltg]

Windows 1.8 failure is unrelated.