fix(burn-in): move psExe detection inside Start-Job scriptblock

Passing $psExe as a second -ArgumentList item caused double-wrapping of
the string array, collapsing all args into one string when splatted.
Computing $IsWindows inside the worker avoids the extra param entirely.

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

scripts/Test-CapacityBurnIn.ps1

@@ -216,18 +216,19 @@ for ($r = 1; $r -le $Rounds; $r++) {
 
         # 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.
+        # $IsWindows is evaluated inside the job worker (correct OS context).
         # 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, [string]$PsExe)
-            $out = & $PsExe @ChildArgs 2>&1
+            param([string[]]$ChildArgs)
+            $psExe = if ($IsWindows) { 'powershell.exe' } else { 'pwsh' }
+            $out = & $psExe @ChildArgs 2>&1
             [PSCustomObject]@{
                 ExitCode = $LASTEXITCODE
                 Output   = ($out -join "`n")
             }
-        } -ArgumentList (, $argArray), $psExe
+        } -ArgumentList (, $argArray)
 
         $jobEntries.Add(@{
             PSJob = $psJob