src: env_vars caching and local variable scope optimization

[mertcanaltin]

I've changed env_vars to variable for caching and tried to improve local variable coverage

[nodejs-github-bot]

Review requested:

• @nodejs/config

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.22%. Comparing base (1123585) to head (2c521ff).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #57624      +/-   ##
==========================================
- Coverage   90.22%   90.22%   -0.01%     
==========================================
  Files         630      630              
  Lines      185055   185056       +1     
  Branches    36216    36221       +5     
==========================================
- Hits       166975   166968       -7     
- Misses      11042    11044       +2     
- Partials     7038     7044       +6     

Files with missing lines	Coverage Δ
src/node_dotenv.cc	85.79% <100.00%> (+0.08%)	⬆️

... and 26 files with indirect coverage changes

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