setup.ps1 emits the "prebuilt installed and validated" / "prebuilt
up to date and validated" markers via the `step` function, which
calls Write-Host. In PowerShell 5+, Write-Host writes to the
Information stream, NOT stdout. Plain `2>&1 | Tee-Object` only
redirects stderr -> stdout, so Information-stream output flows to
the host (visible in the GitHub Actions log) but never lands in
logs/install.log. The post-step grep asserter then fails with
"no Windows prebuilt llama.cpp marker in install.log" even though
the prebuilt was installed correctly.
Switch to `*>&1` (the wildcard "all streams" redirect) so
Tee-Object captures Information stream too. Also silence the
ProgressPreference noise that fills install.log with progress-bar
ANSI sequences.
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