Fix loading netrc when NETRC env var is not set

[bdraco]

What do these changes do?

I optimized this one a bit too much assuming NETRC would always be set in #11634

fixes #11713

Are there changes in behavior for the user?

fix regression

Is it a substantial burden for the maintainers to support this?

no

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.74%. Comparing base (1569ad9) to head (654d7e6).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #11714   +/-   ##
=======================================
  Coverage   98.74%   98.74%           
=======================================
  Files         127      127           
  Lines       43549    43573   +24     
  Branches     2327     2327           
=======================================
+ Hits        43003    43027   +24     
  Misses        390      390           
  Partials      156      156           

Flag	Coverage Δ
CI-GHA	98.63% <100.00%> (+<0.01%)	⬆️
OS-Linux	98.36% <100.00%> (+<0.01%)	⬆️
OS-Windows	96.69% <100.00%> (-0.01%)	⬇️
OS-macOS	97.58% <100.00%> (-0.01%)	⬇️
Py-3.10.11	97.15% <100.00%> (-0.01%)	⬇️
Py-3.10.18	97.65% <100.00%> (+<0.01%)	⬆️
Py-3.11.13	97.85% <100.00%> (+<0.01%)	⬆️
Py-3.11.9	97.35% <100.00%> (+<0.01%)	⬆️
Py-3.12.10	97.45% <100.00%> (+<0.01%)	⬆️
Py-3.12.11	97.95% <100.00%> (-0.01%)	⬇️
Py-3.13.7	97.95% <100.00%> (+<0.01%)	⬆️
Py-3.13.8	97.44% <100.00%> (+0.01%)	⬆️
Py-3.14.0	98.15% <100.00%> (-0.01%)	⬇️
Py-3.14.0t	97.23% <100.00%> (+<0.01%)	⬆️
Py-pypy3.10.16-7.3.19	90.54% <100.00%> (-2.96%)	⬇️
VM-macos	97.58% <100.00%> (-0.01%)	⬇️
VM-ubuntu	98.36% <100.00%> (+<0.01%)	⬆️
VM-windows	96.69% <100.00%> (-0.01%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #11714 will not alter performance

_{Comparing netrc_without_env_fix (654d7e6) with master (1569ad9)¹}

Summary

✅ 59 untouched

Footnotes

1. No successful run was found on master (6a2809d) during the generation of this report, so 1569ad9 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩