Run the Windows installer under PowerShell 5.1, the only shell a clean machine has

The Windows Install step ran `& $script` inside a `shell: pwsh` step, so
install.ps1 was executing under PowerShell 7. A genuinely clean Windows box
does not have PowerShell 7: Windows ships powershell.exe (Windows PowerShell
5.1) and pwsh is a separate install that the hosted runner image happens to
preinstall. So the one workflow whose premise is a machine that has never seen
a developer toolchain was testing the installer under a shell that machine
would not have, and no other Windows job anywhere in .github exercises
install.ps1 under 5.1.

Invoke it the way the desktop does (install.rs:325-339, and the bundled
installer step in desktop-app-clean-machine-ci.yml): powershell.exe with
-NoLogo -NoProfile -NonInteractive -ExecutionPolicy Bypass -File. The pwsh
step wrapper stays, since it is only the installer that has to be under 5.1.
Calling powershell.exe with `&` keeps the output in the pipeline, so
Tee-Object still fills logs/install.log, and $LASTEXITCODE after the pipeline
is the child's real exit code, so $rc and `exit $rc` are unchanged.

install.ps1 and studio/setup.ps1 hold no PowerShell 7-only constructs: no
`#Requires` above 5.1, no `&&`/`||` chain operators, no ternary, no
null-coalescing, no ForEach-Object -Parallel, no $IsWindows/$PSStyle, and no
6+ cmdlets or parameters. setup.ps1 declares `#Requires -Version 5.1`, and its
three $PSVersionTable branches gate a 7-only preference on the 7 side with a
5.1 fallback. Every Invoke-WebRequest already passes -UseBasicParsing, which
5.1 needs because it otherwise reaches for the IE engine.

.github/workflows/clean-machine-install-ci.yml

@@ -897,7 +897,11 @@ jobs:
           # `--no-torch` only (112-142), so the token was silently dropped and every
           # Windows leg installed torch anyway. The assert below needs torch, so get it
           # on purpose rather than by accident.
-          & $script *>&1 | Tee-Object -FilePath logs/install.log
+          # Under powershell.exe, not this pwsh 7 step: a clean Windows box ships
+          # Windows PowerShell 5.1 only, and the desktop launches it the same way
+          # (install.rs:325-339). pwsh 7 is a runner-image extra no user is promised.
+          & powershell.exe -NoLogo -NoProfile -NonInteractive -ExecutionPolicy Bypass `
+              -File $script *>&1 | Tee-Object -FilePath logs/install.log
           $rc = $LASTEXITCODE
           Write-Host "installer exit code: $rc"
           exit $rc