src: swap dotenv and config file parsing order

[marco-ippolito]

Makes more sense that env variable override config file and not the opposite

[nodejs-github-bot]

Review requested:

• @nodejs/config
• @nodejs/startup

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.63%. Comparing base (8173251) to head (e7ed568).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63035      +/-   ##
==========================================
- Coverage   91.49%   89.63%   -1.86%     
==========================================
  Files         358      708     +350     
  Lines      151574   220375   +68801     
  Branches    23921    42270   +18349     
==========================================
+ Hits       138679   197539   +58860     
- Misses      12619    14713    +2094     
- Partials      276     8123    +7847     

Files with missing lines	Coverage Δ
src/node.cc	76.15% <100.00%> (ø)
src/node_dotenv.cc	84.72% <100.00%> (ø)

... and 469 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73022/