ci: use Powershell 7+ for Windows workflow

[dunglas]

No description provided.

[Copilot]

Pull request overview

This PR removes the explicit shell: powershell (Windows PowerShell 5.1) default from the Windows CI workflow, allowing it to use the GitHub Actions default of pwsh (PowerShell 7+). The accompanying changes update idioms that were specific to Windows PowerShell 5.1 to their cleaner PowerShell 7+ equivalents.

Changes:

• Removed the defaults.run.shell: powershell block, switching to pwsh (PowerShell 7+) as the default shell
• Replaced echo "..." | Out-File -FilePath ... -Append with the simpler "..." >> $env:GITHUB_ENV / $env:GITHUB_PATH redirection syntax and removed the unnecessary -UseBasicParsing flag from Invoke-WebRequest
• Replaced [System.IO.File]::WriteAllText(...) with Set-Content for writing the JSON metadata file

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