Two Windows-guest build bugs:
- The build command was wrapped in `cmd /c`, so PowerShell-authored
commands failed ("'New-Item' is not recognized"). Run it as PS
(seed LASTEXITCODE=0 so PS-only commands exit cleanly).
- artifact_source was always Join-Path'd with the workdir; an absolute
source (C:\CI\output) became C:\CI\build\C:\CI\output. Use the path
as-is when rooted, else Join-Path the workdir.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Simone
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