fix #10422: add test for env_var casing on windows

[aranke]

Resolves #10422

Problem

• dbt-core 1.8+ is treating env-vars are case-sensitive on Windows, even though Windows env-vars are not case-sensitive.
• This is a regression from dbt-core 1.7, since the test I wrote passes with 1.6.
• I believe this is the PR that introduced the change: Use new context invocation class to cache environment variables.  #9489

Solution

• I have a failing test
• I tried a code fix, but didn't work, so need some guidance here @peterallenwebb

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.82%. Comparing base (4c7d922) to head (521c485).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10436      +/-   ##
==========================================
+ Coverage   88.76%   88.82%   +0.05%     
==========================================
  Files         180      180              
  Lines       22564    22564              
==========================================
+ Hits        20029    20042      +13     
+ Misses       2535     2522      -13     

Flag	Coverage Δ
integration	86.20% <ø> (+0.16%)	⬆️
unit	62.10% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.10% <ø> (ø)
Integration Tests	86.20% <ø> (+0.16%)	⬆️