[macOS] Bugfix for Xcode installer script

[erik-bershel]

Description

Fix using undeclared variable

Related issue:

Check list

• Related issue / work item is attached
• Tests are written (if applicable)
• Documentation is updated (if applicable)
• Changes are tested and related VM images are successfully generated

[Copilot]

Pull Request Overview

This PR fixes a bug in the Xcode installer script where an undeclared variable was being used. The fix adds the missing declaration for the $os variable.

• Adds declaration for the $os variable using Get-OSVersion function

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