Avoid evaluating $IsWindows under Strict mode in PS 5.1

PS 5.1 does not define the $IsWindows automatic variable, and the
hidden-folders tests run under Strict mode, so reading it directly
threw 'The variable $IsWindows cannot be retrieved because it has
not been set' in the PS_5_1_Windows_Server2022 leg.

Reorder the check so the version comparison short-circuits before
$IsWindows is touched: in PS 5.1 the first operand is $true and
$IsWindows is never evaluated; in PS 6+ the variable is always set
so reading it is safe.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nohwnd

tst/Pester.RSpec.ts.ps1

@@ -2980,7 +2980,9 @@ i -PassThru:$PassThru {
             $null = New-Item -ItemType Directory -Path $hiddenDir -Force
             # On Windows the dot-prefix alone is not enough — mark it Hidden so
             # Get-ChildItem actually treats it as hidden (matches Linux behavior).
-            if ($IsWindows -or $env:OS -eq 'Windows_NT') {
+            # Use a short-circuit check so $IsWindows is not evaluated under Strict
+            # mode in PowerShell 5.1 (where the automatic variable does not exist).
+            if (($PSVersionTable.PSVersion.Major -lt 6) -or $IsWindows) {
                 $di = Get-Item -LiteralPath $hiddenDir -Force
                 $di.Attributes = $di.Attributes -bor [System.IO.FileAttributes]::Hidden
             }
@@ -3001,7 +3003,7 @@ i -PassThru:$PassThru {
             $tempDir = Join-Path ([IO.Path]::GetTempPath()) "PesterHidden2_$([IO.Path]::GetRandomFileName().Substring(0,8))"
             $nestedHidden = Join-Path $tempDir '.config/tests'
             $null = New-Item -ItemType Directory -Path $nestedHidden -Force
-            if ($IsWindows -or $env:OS -eq 'Windows_NT') {
+            if (($PSVersionTable.PSVersion.Major -lt 6) -or $IsWindows) {
                 $dotConfig = Get-Item -LiteralPath (Join-Path $tempDir '.config') -Force
                 $dotConfig.Attributes = $dotConfig.Attributes -bor [System.IO.FileAttributes]::Hidden
             }