ci: install Python for Qt setup on Windows

.gitea/workflows/build-windows-release.yml

@@ -67,9 +67,23 @@ jobs:
             $pythonArgs = @('-3')
           } else {
             $python = Get-Command python -ErrorAction SilentlyContinue
-            if (-not $python) { throw 'Python is required to install Qt with aqtinstall, but py/python was not found on PATH.' }
+            if ($python) {
               $pythonExe = $python.Source
               $pythonArgs = @()
+            } else {
+              $pythonRoot = Join-Path $env:RUNNER_TEMP 'python312'
+              $pythonExe = Join-Path $pythonRoot 'python.exe'
+              if (-not (Test-Path $pythonExe)) {
+                $installer = Join-Path $env:RUNNER_TEMP 'python-installer.exe'
+                $pythonUrl = 'https://www.python.org/ftp/python/3.12.10/python-3.12.10-amd64.exe'
+                Write-Host "Downloading Python from $pythonUrl"
+                Invoke-WebRequest -Uri $pythonUrl -OutFile $installer
+                & $installer /quiet InstallAllUsers=0 TargetDir=$pythonRoot Include_pip=1 Include_test=0 PrependPath=0
+                if ($LASTEXITCODE -ne 0) { throw "Python installer failed, exit code: $LASTEXITCODE" }
+              }
+              if (-not (Test-Path $pythonExe)) { throw "Python installation finished but python.exe was not found: $pythonExe" }
+              $pythonArgs = @()
+            }
           }
 
           & $pythonExe @pythonArgs -m pip install --user --upgrade aqtinstall