Allow associating env vars with flags

[FollowTheProcess]

Summary

[codecov]

Codecov Report

❌ Patch coverage is 85.96491% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.15%. Comparing base (dd90a26) to head (3c197fd).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/flag/flag.go	73.33%	3 Missing and 1 partial ⚠️
internal/flag/set.go	91.66%	1 Missing and 1 partial ⚠️
option.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
- Coverage   85.29%   85.15%   -0.15%     
==========================================
  Files           9        9              
  Lines        1578     1617      +39     
==========================================
+ Hits         1346     1377      +31     
- Misses        167      172       +5     
- Partials       65       68       +3     

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