Update links in about_Environment_Variables.md

[derekm]

about_*.md links are case-sensitive on sane platforms

PR Summary

Some links in about_Environment_Variables.md were entered as all-lowercase, even though the filenames contain capital letters. On saner platforms, filenames are case-sensitive. One such platform is GitHub here, where traversing doc links produces 404s.

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributors guide.
• Style: This PR adheres to the style guide.

[learn-build-service-prod]

Learn Build status updates of commit cbcd6d7:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-5.1)
reference/7.4/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-7.4)
reference/7.6/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-7.6)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[github-actions]

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[sdwheeler]

This PR was made obsolete by PR #11724, which was already in progress.

[learn-build-service-prod]

Learn Build status updates of commit 0c64386:

✅ Validation status: passed

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel