[nextest-runner] read build script env vars from structured Cargo data

[sunshowers]

Part 1 of fixes for #3167. Don't depend on Cargo internals since a stable interface is available.

We no longer archive the output file, so this breaks new archives + old nextest, but that should be okay: in general I'd expect old archive + new nextest to work better.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.19%. Comparing base (790bd81) to head (a730ece).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/test_command.rs	70.00%	3 Missing ⚠️
nextest-runner/src/list/binary_list.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3168      +/-   ##
==========================================
- Coverage   85.20%   85.19%   -0.02%     
==========================================
  Files         158      158              
  Lines       46366    46399      +33     
==========================================
+ Hits        39508    39528      +20     
- Misses       6858     6871      +13     

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[trunk-io]

     

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