(WIP) Add env= arg to tyro.conf.arg()

[brentyi]

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[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.42%. Comparing base (8e773a4) to head (febdb51).

Files with missing lines	Patch %	Lines
src/tyro/_arguments.py	50.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #456      +/-   ##
==========================================
- Coverage   99.64%   99.42%   -0.23%     
==========================================
  Files          52       52              
  Lines        3984     4005      +21     
==========================================
+ Hits         3970     3982      +12     
- Misses         14       23       +9     

Flag	Coverage Δ
unittests	99.42% <57.14%> (-0.23%)	⬇️

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