fix(burn-in): use pwsh instead of powershell.exe on Linux host

powershell.exe does not exist on Linux — child process launch failed
instantly (ExitCode null, Elapsed 00:00). Now selects powershell.exe
on Windows and pwsh on Linux via $IsWindows automatic variable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/Test-CapacityBurnIn.ps1

@@ -214,19 +214,20 @@ for ($r = 1; $r -le $Rounds; $r++) {
 
         $argArray = $argList.ToArray()
 
-        # Start-Job runs a new PS session; call powershell.exe as child process
-        # so exit $exitCode from Invoke-CIJob.ps1 is captured in $LASTEXITCODE.
+        # Start-Job runs a new PS session; call the platform PS executable as child
+        # process so exit $exitCode from Invoke-CIJob.ps1 is captured in $LASTEXITCODE.
         # NOTE: do NOT name the parameter $Args — @Args is a PS reserved splatting
         # alias for the automatic $Args variable (unbound arguments), which would
         # be empty here since the parameter binding consumed all positional args.
+        $psExe = if ($IsWindows) { 'powershell.exe' } else { 'pwsh' }
         $psJob = Start-Job -ScriptBlock {
-            param([string[]]$ChildArgs)
-            $out = & powershell.exe @ChildArgs 2>&1
+            param([string[]]$ChildArgs, [string]$PsExe)
+            $out = & $PsExe @ChildArgs 2>&1
             [PSCustomObject]@{
                 ExitCode = $LASTEXITCODE
                 Output   = ($out -join "`n")
             }
-        } -ArgumentList (, $argArray)
+        } -ArgumentList (, $argArray), $psExe
 
         $jobEntries.Add(@{
             PSJob = $psJob