All shims defaulted to F:\CI\python\venv\Scripts\python.exe when
CI_VENV_PYTHON was not set. On Linux host this caused instant failure.
Now uses $IsWindows (PS 6+ automatic var; absent on PS 5.1 = Windows)
to select /opt/ci/venv/bin/python on Linux. Windows paths unchanged.
Affects: Invoke-CIJob, Get-BuildArtifacts, Cleanup-OrphanedBuildVMs,
New-BuildVM, Remove-BuildVM, Wait-VMReady, Get-CIJobSummary,
Invoke-RemoteBuild, Watch-DiskSpace, Watch-RunnerHealth,
Set-CIGuestCredential, Test-CIGuestWinRM.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Simone
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