Fix case-insensitive env vars for Windows

[aranke]

Description

Add a new CaseInsensitiveMapping class and use that for env vars on Windows.
Add a test to verify this case-insensitive mapping.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have signed the CLA
• I have run this code in development, and it appears to resolve the stated issue
• I have opened an issue to add/update docs, or docs changes are not required/relevant for this PR
• I have run changie new to create a changelog entry

[github-actions]

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[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-commenter]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes missing coverage. Please review.

Project coverage is 64.34%. Comparing base (6e3828e) to head (16f61ba).

Files	Patch %	Lines
dbt_common/context.py	73.91%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   64.31%   64.34%   +0.03%     
==========================================
  Files          51       51              
  Lines        3203     3223      +20     
==========================================
+ Hits         2060     2074      +14     
- Misses       1143     1149       +6     

Flag	Coverage Δ
unit	64.34% <73.91%> (+0.03%)	⬆️

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