Extract Get-HostMachineArch for the VC++ round-trip test (#7597)

#7549 taught Test-VCRedistInstalled to consult the host architecture before
trusting the System32 DLL, but the round-trip job dot-sources a fixed list of
functions out of setup.ps1 and that list did not gain the helper. Part A returns
early on the registry hit, so only the clean-box half reaches the call and the
job fails there with "Get-HostMachineArch is not recognized".

Reproduced by dot-sourcing the old list and calling Test-VCRedistInstalled, which
throws; with the helper added the same call returns.

.github/workflows/studio-windows-inference-smoke.yml

@@ -1888,8 +1888,11 @@ jobs:
           # (step/substep -> Write-StudioStdoutMirror / Get-StudioAnsi).
           $script:StudioVtOk = $false
           $script:UnslothVerbose = $false
+          # Get-HostMachineArch is reached only on the absent path, where
+          # Test-VCRedistInstalled consults it before trusting the System32 DLL, so
+          # part A passes without it and only the clean-box part fails.
           foreach ($fn in @('Get-StudioAnsi', 'Write-StudioStdoutMirror', 'step', 'substep',
-                            'Invoke-SetupCommand', 'Refresh-Environment',
+                            'Invoke-SetupCommand', 'Refresh-Environment', 'Get-HostMachineArch',
                             'Test-VCRedistInstalled', 'Ensure-VCRedist')) {
               $src = Get-FunctionSource -Path $setup -Name $fn
               if (-not $src) { throw "Function '$fn' not found in setup.ps1" }