[nextest-runner] even more setup script prep

[sunshowers]

Move some code around, and a small fix.

[codecov]

Codecov Report

Merging #976 (34f8aa5) into main (61624f8) will decrease coverage by 0.01%.
The diff coverage is 61.90%.

@@            Coverage Diff             @@
##             main     #976      +/-   ##
==========================================
- Coverage   78.11%   78.10%   -0.01%     
==========================================
  Files          64       64              
  Lines       14891    14903      +12     
==========================================
+ Hits        11632    11640       +8     
- Misses       3259     3263       +4     

Files	Coverage Δ
nextest-runner/src/config/overrides.rs	94.64% <ø> (-0.03%)	⬇️
nextest-runner/src/config/test_helpers.rs	97.72% <100.00%> (+0.16%)	⬆️
nextest-runner/src/reporter.rs	52.05% <55.55%> (+0.22%)	⬆️

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