fix(ci): use cross-platform temp dir in fetch-ontario-design-system.ps1

$env:TEMP is undefined on Linux GitHub runners, which made the build
job fail at the 'Fetch Ontario Design System assets' step with
'Cannot bind argument to parameter Path because it is null'.

Switch to [System.IO.Path]::GetTempPath() which resolves correctly on
Windows, macOS, and Linux pwsh.

Author: emmanuelknafo

scripts/fetch-ontario-design-system.ps1

@@ -50,7 +50,9 @@ if (-not (Test-Path $AbsVendorDir)) {
 }
 
 $zipUrl  = "https://designsystem.ontario.ca/dist/ontario-design-system-dist-$Version.zip"
-$zipPath = Join-Path $env:TEMP "ontario-design-system-$Version.zip"
+# Use [System.IO.Path]::GetTempPath() so this works on Linux GitHub runners
+# (where $env:TEMP is undefined) as well as Windows/macOS.
+$zipPath = Join-Path ([System.IO.Path]::GetTempPath()) "ontario-design-system-$Version.zip"
 
 if (Test-Path $zipPath) { Remove-Item $zipPath -Force }
 $ProgressPreference = 'SilentlyContinue'