provisioner/powershell: Use system context env var to set output file path

Fixes #4271

provisioner/powershell/elevated.go

@@ -14,7 +14,7 @@ type elevatedOptions struct {
 
 var elevatedTemplate = template.Must(template.New("ElevatedCommand").Parse(`
 $name = "{{.TaskName}}"
-$log = "$env:TEMP\$name.out"
+$log = "$env:SystemRoot\Temp\$name.out"
 $s = New-Object -ComObject "Schedule.Service"
 $s.Connect()
 $t = $s.NewTask($null)
@@ -53,7 +53,7 @@ $t.XmlText = @'
   <Actions Context="Author">
     <Exec>
       <Command>cmd</Command>
-	  <Arguments>/c powershell.exe -EncodedCommand {{.EncodedCommand}} &gt; %TEMP%\{{.TaskName}}.out 2&gt;&amp;1</Arguments>
+	  <Arguments>/c powershell.exe -EncodedCommand {{.EncodedCommand}} &gt; %SYSTEMROOT%\Temp\{{.TaskName}}.out 2&gt;&amp;1</Arguments>
     </Exec>
   </Actions>
 </Task>
@@ -81,5 +81,12 @@ do {
   }
 } while (!($t.state -eq 3))
 $result = $t.LastTaskResult
+if (Test-Path $log) {
+  try {
+    Takeown /F $log | Out-Null
+    Icacls $log /Grant:r Administrators:F /c /q 2>&1 | Out-Null
+    Remove-Item $log -Force -ErrorAction SilentlyContinue | Out-Null
+  } catch { $Global:Error.RemoveAt(0) }
+}
 [System.Runtime.Interopservices.Marshal]::ReleaseComObject($s) | Out-Null
 exit $result`))