ci(welcome): pin .NET 9 SDK via setup-dotnet so MAUI workload band matches

.gitea/workflows/build-iso-windows.yaml

@@ -58,18 +58,15 @@ jobs:
           }
           if (-not (Test-Path $deploy)) { throw 'ADK Deployment Tools install failed.' }
 
-      - name: Ensure .NET 9 SDK + MAUI workload
+      - name: Setup .NET 9 SDK
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: '9.0.x'
+
+      - name: Install MAUI workload
         shell: pwsh
         run: |
-          # Ensure dotnet 9 is available (setup-dotnet may have cached it; self-hosted
-          # runners should already have it, but this makes the step idempotent).
-          $dotnet = Get-Command dotnet -EA SilentlyContinue
-          if ($dotnet) {
-            $ver = (dotnet --version 2>$null) -replace '\+.*',''
-            Write-Host "dotnet found: $ver"
-          } else {
-            throw 'dotnet not found on runner. Add actions/setup-dotnet@v4 to this job.'
-          }
+          Write-Host "dotnet $(dotnet --version)"
           Write-Host 'Installing/repairing MAUI workload (idempotent)...'
           dotnet workload install maui
           Write-Host 'MAUI workload ready.'