ci(lint): use actions/setup-python to provision Python on runner

The act_runner service account doesn't have python in PATH. Delegating
to actions/setup-python@v5 which downloads/caches Python 3.11 reliably.

Also bump timeout-minutes 15 -> 20 to absorb first-run pip install.

.gitea/workflows/lint.yml

@@ -62,7 +62,7 @@ jobs:
 
   python:
     runs-on: windows-build
-    timeout-minutes: 15
+    timeout-minutes: 20
 
     env:
       PYTHONIOENCODING: utf-8
@@ -74,12 +74,20 @@ jobs:
         with:
           fetch-depth: 1
 
+      - name: Setup Python 3.11
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
       - name: Setup venv + install package
         shell: powershell
         run: |
           $ErrorActionPreference = 'Stop'
 
+          # python is provided by actions/setup-python and is on PATH for this step.
           $py = (Get-Command python -ErrorAction Stop).Source
+          Write-Host "Using python: $py"
+
           if (-not (Test-Path "$env:VENV_DIR\Scripts\python.exe")) {
             Write-Host "Creating venv at $env:VENV_DIR"
             & $py -m venv $env:VENV_DIR