Change ‘WINDIR’ to ‘SYSTEMROOT’

[gaelicWizard]

Pull Request (PR) description

Use environment variable SystemRoot over WINDIR...just because I think that's what we're supposed to use these days? Please tell me if this is not the kind of change that is useful and I'll just discard it.

This Pull Request (PR) fixes the following issues

Task list

• Added an entry under the Unreleased section of the change log in the CHANGELOG.md.
  Entry should say what was changed, and how that affects users (if applicable).
• Resource documentation added/updated in README.md in resource folder.
• Resource parameter descriptions added/updated in schema.mof
  and comment-based help.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Resource Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97%. Comparing base (983c54b) to head (759522b).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #497   +/-   ##
===================================
  Coverage    97%    97%           
===================================
  Files        28     28           
  Lines      2070   2070           
===================================
  Hits       2021   2021           
  Misses       49     49           

Files with missing lines	Coverage Δ
...urce/DSCResources/DSC_HostsFile/DSC_HostsFile.psm1	97% <100%> (ø)

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